diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
index db80b49e8a..af8035eea7 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
@@ -37,8 +37,7 @@ Foam::IPstream::IPstream
     const label bufSize,
     const int tag,
     const label comm,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     Pstream(commsType, bufSize),
@@ -46,15 +45,14 @@ Foam::IPstream::IPstream
     (
         commsType,
         fromProcNo,
-        buf_,
-        externalBufPosition_,
-        tag,                        // tag
+        Pstream::transferBuf_,
+        transferBufPosition_,
+        tag,
         comm,
-        false,                      // do not clear buf_ if at end
-        fmt,
-        ver
+        false,  // Do not clear Pstream::transferBuf_ if at end
+        fmt
     ),
-    externalBufPosition_(0)
+    transferBufPosition_(0)
 {}
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index fe42f3c2a2..5caea64dc9 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -56,16 +56,17 @@ class IPstream
     public Pstream,
     public UIPstream
 {
+    // Private Data
 
-    //- Receive index
-    label externalBufPosition_;
+        //- Receive index into Pstream::transferBuf_
+        label transferBufPosition_;
 
 public:
 
     // Constructors
 
-        //- Construct given process index to read from and optional buffer size,
-        //- read format and IO version
+        //- Construct given process index to read from
+        //- and optional buffer size, read format
         IPstream
         (
             const commsTypes commsType,
@@ -73,8 +74,7 @@ public:
             const label bufSize = 0,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm,
-            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY,
-            IOstreamOption::versionNumber ver = IOstreamOption::currentVersion
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 };
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
index ee1322fdc0..75805c718d 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
@@ -37,12 +37,20 @@ Foam::OPstream::OPstream
     const label bufSize,
     const int tag,
     const label comm,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     Pstream(commsType, bufSize),
-    UOPstream(commsType, toProcNo, buf_, tag, comm, true, fmt, ver)
+    UOPstream
+    (
+        commsType,
+        toProcNo,
+        Pstream::transferBuf_,
+        tag,
+        comm,
+        true,  // sendAtDestruct
+        fmt
+    )
 {}
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
index 575cb048ba..c91c230377 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
@@ -60,8 +60,8 @@ public:
 
     // Constructors
 
-        //- Construct given process index to send to and optional buffer size,
-        //- write format and IO version
+        //- Construct for given process index to send to
+        //- and optional buffer size, write format
         OPstream
         (
             const commsTypes commsType,
@@ -69,8 +69,7 @@ public:
             const label bufSize = 0,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm,
-            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY,
-            IOstreamOption::versionNumber ver = IOstreamOption::currentVersion
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 };
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index 1c8ad9e618..6ae66c7c0a 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -32,8 +32,8 @@ Description
 
 SourceFiles
     Pstream.C
-    gatherScatter.C
     combineGatherScatter.C
+    gatherScatter.C
     gatherScatterList.C
     exchange.C
 
@@ -92,10 +92,10 @@ class Pstream
 
 protected:
 
-    // Protected data
+    // Protected Data
 
-        //- Transfer buffer
-        DynamicList<char> buf_;
+        //- Allocated transfer buffer (can be used for send or receive)
+        DynamicList<char> transferBuf_;
 
 
 public:
@@ -107,26 +107,25 @@ public:
     // Constructors
 
         //- Construct given optional buffer size
-        Pstream
+        explicit Pstream
         (
             const commsTypes commsType,
             const label bufSize = 0
         )
         :
-            UPstream(commsType),
-            buf_(0)
+            UPstream(commsType)
         {
             if (bufSize)
             {
-                buf_.setCapacity(bufSize + 2*sizeof(scalar) + 1);
+                transferBuf_.setCapacity(bufSize + 2*sizeof(scalar) + 1);
             }
         }
 
 
         // Gather and scatter
 
-            //- Gather data. Apply bop to combine Value
-            //  from different processors
+            //- Gather data.
+            //- Apply bop to combine Value from different processors
             template<class T, class BinaryOp>
             static void gather
             (
@@ -166,6 +165,7 @@ public:
                 const label comm = Pstream::worldComm
             );
 
+
         // Combine variants. Inplace combine values from processors.
         // (Uses construct from Istream instead of <<)
 
@@ -208,6 +208,7 @@ public:
                 const label comm = Pstream::worldComm
             );
 
+
         // Combine variants working on whole List at a time.
 
             template<class T, class CombineOp>
@@ -249,6 +250,7 @@ public:
                 const label comm = Pstream::worldComm
             );
 
+
         // Combine variants working on whole map at a time. Container needs to
         // have iterators and find() defined.
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
index 117e7f505d..2607ce97ab 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
@@ -28,11 +28,6 @@ License
 
 #include "PstreamBuffers.H"
 
-/* * * * * * * * * * * * * * * Static Member Data  * * * * * * * * * * * * * */
-
-Foam::DynamicList<char> Foam::PstreamBuffers::nullBuf(0);
-
-
 // * * * * * * * * * * * * * * * * Constructor * * * * * * * * * * * * * * * //
 
 Foam::PstreamBuffers::PstreamBuffers
@@ -40,18 +35,16 @@ Foam::PstreamBuffers::PstreamBuffers
     const UPstream::commsTypes commsType,
     const int tag,
     const label comm,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     commsType_(commsType),
     tag_(tag),
     comm_(comm),
     format_(fmt),
-    version_(ver),
     sendBuf_(UPstream::nProcs(comm)),
     recvBuf_(UPstream::nProcs(comm)),
-    recvBufPos_(UPstream::nProcs(comm), 0),
+    recvBufPos_(UPstream::nProcs(comm), Zero),
     finishedSendsCalled_(false)
 {}
 
@@ -67,9 +60,8 @@ Foam::PstreamBuffers::~PstreamBuffers()
         {
             FatalErrorInFunction
                 << "Message from processor " << proci
-                << " not fully consumed. messageSize:" << recvBuf_[proci].size()
-                << " bytes of which only " << recvBufPos_[proci]
-                << " consumed."
+                << " Only consumed " << recvBufPos_[proci] << " of "
+                << recvBuf_[proci].size() << " bytes" << nl
                 << Foam::abort(FatalError);
         }
     }
@@ -80,6 +72,7 @@ Foam::PstreamBuffers::~PstreamBuffers()
 
 void Foam::PstreamBuffers::finishedSends(const bool block)
 {
+    // Could also check that it is not called twice
     finishedSendsCalled_ = true;
 
     if (commsType_ == UPstream::commsTypes::nonBlocking)
@@ -98,6 +91,7 @@ void Foam::PstreamBuffers::finishedSends(const bool block)
 
 void Foam::PstreamBuffers::finishedSends(labelList& recvSizes, const bool block)
 {
+    // Could also check that it is not called twice
     finishedSendsCalled_ = true;
 
     if (commsType_ == UPstream::commsTypes::nonBlocking)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
index 072ce891da..de7414f469 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
@@ -91,79 +91,79 @@ class PstreamBuffers
     friend class UOPstream;
     friend class UIPstream;
 
-    // Private data
+    // Private Data
 
         //- Communications type of this stream
         const UPstream::commsTypes commsType_;
 
+        //- The transfer message type
         const int tag_;
 
+        //- Communicator
         const label comm_;
 
+        //- Buffer format (ascii | binary)
         const IOstreamOption::streamFormat format_;
 
-        const IOstreamOption::versionNumber version_;
-
         //- Send buffer
         List<DynamicList<char>> sendBuf_;
 
         //- Receive buffer
         List<DynamicList<char>> recvBuf_;
 
-        //- Read position in recvBuf_
+        //- Current read positions within recvBuf_
         labelList recvBufPos_;
 
+        //- Track if sends are complete
         bool finishedSendsCalled_;
 
+
 public:
 
-    // Static data
-
-        static DynamicList<char> nullBuf;
-
-
     // Constructors
 
-        //- Construct given comms type,
-        //  write format and IO version
-        PstreamBuffers
+        //- Construct given comms type, communication options, IO format
+        explicit PstreamBuffers
         (
             const UPstream::commsTypes commsType,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm,
-            IOstreamOption::streamFormat vmt = IOstreamOption::BINARY,
-            IOstreamOption::versionNumber ver = IOstreamOption::currentVersion
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
-    //- Destructor
+
+    //- Destructor - checks that all data have been consumed
     ~PstreamBuffers();
 
 
-    // Member functions
+    // Member Functions
 
-        int tag() const
+        //- The transfer message type
+        int tag() const noexcept
         {
             return tag_;
         }
 
-        label comm() const
+        //- Communicator
+        label comm() const noexcept
         {
             return comm_;
         }
 
-        //- Mark all sends as having been done. This will start receives
-        //  in non-blocking mode. If block will wait for all transfers to
-        //  finish (only relevant for nonBlocking mode)
+        //- Mark all sends as having been done.
+        //  This will start receives in non-blocking mode.
+        //  If block will wait for all transfers to finish
+        //  (only relevant for nonBlocking mode)
         void finishedSends(const bool block = true);
 
-        //- Mark all sends as having been done. Same as above but also returns
-        //  sizes (bytes) received. Note:currently only valid for
-        //  non-blocking.
+        //- Mark all sends as having been done.
+        //  Same as above but also returns sizes (bytes) received.
+        //  \note currently only valid for non-blocking.
         void finishedSends(labelList& recvSizes, const bool block = true);
 
-        //- Clear storage and reset
+        //- Reset (clear) individual buffers and reset state.
+        //  Does not clear buffer storage
         void clear();
-
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
index f999a3f093..5d8187da08 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
@@ -57,14 +57,26 @@ inline static void processFlags(Istream& is, int flagMask)
     }
 }
 
-} // End anonymous namespace
+
+// Return the position with word boundary alignment
+inline static label byteAlign(const label pos, const size_t align)
+{
+    return
+    (
+        (align > 1)
+      ? (align + ((pos - 1) & ~(align - 1)))
+      : pos
+    );
+}
+
+} // End namespace Foam
 
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
 inline void Foam::UIPstream::checkEof()
 {
-    if (externalBufPosition_ == messageSize_)
+    if (recvBufPos_ == messageSize_)
     {
         setEof();
     }
@@ -73,11 +85,7 @@ inline void Foam::UIPstream::checkEof()
 
 inline void Foam::UIPstream::prepareBuffer(const size_t align)
 {
-    if (align > 1)
-    {
-        externalBufPosition_ =
-            align + ((externalBufPosition_ - 1) & ~(align - 1));
-    }
+    recvBufPos_ = byteAlign(recvBufPos_, align);
 }
 
 
@@ -86,8 +94,8 @@ inline void Foam::UIPstream::readFromBuffer(T& val)
 {
     prepareBuffer(sizeof(T));
 
-    val = reinterpret_cast<T&>(externalBuf_[externalBufPosition_]);
-    externalBufPosition_ += sizeof(T);
+    val = reinterpret_cast<T&>(recvBuf_[recvBufPos_]);
+    recvBufPos_ += sizeof(T);
     checkEof();
 }
 
@@ -98,7 +106,7 @@ inline void Foam::UIPstream::readFromBuffer
     const size_t count
 )
 {
-    const char* const __restrict__ buf = &externalBuf_[externalBufPosition_];
+    const char* const __restrict__ buf = &recvBuf_[recvBufPos_];
     char* const __restrict__ output = reinterpret_cast<char*>(data);
 
     for (size_t i = 0; i < count; ++i)
@@ -106,12 +114,12 @@ inline void Foam::UIPstream::readFromBuffer
         output[i] = buf[i];
     }
 
-    externalBufPosition_ += count;
+    recvBufPos_ += count;
     checkEof();
 }
 
 
-inline Foam::Istream& Foam::UIPstream::readStringFromBuffer(std::string& str)
+inline Foam::Istream& Foam::UIPstream::readString(std::string& str)
 {
     // Use std::string::assign() to copy content, including '\0'.
     // Stripping (when desired) is the responsibility of the sending side.
@@ -121,8 +129,8 @@ inline Foam::Istream& Foam::UIPstream::readStringFromBuffer(std::string& str)
 
     if (len)
     {
-        str.assign(&externalBuf_[externalBufPosition_], len);
-        externalBufPosition_ += len;
+        str.assign(&recvBuf_[recvBufPos_], len);
+        recvBufPos_ += len;
         checkEof();
     }
     else
@@ -144,11 +152,11 @@ Foam::UIPstream::~UIPstream()
         {
             Pout<< "UIPstream::~UIPstream() : tag:" << tag_
                 << " fromProcNo:" << fromProcNo_
-                << " clearing externalBuf_ of size "
-                << externalBuf_.size()
+                << " clearing receive buffer of size "
+                << recvBuf_.size()
                 << " messageSize_:" << messageSize_ << endl;
         }
-        externalBuf_.clearStorage();
+        recvBuf_.clearStorage();
     }
 }
 
@@ -232,7 +240,7 @@ Foam::Istream& Foam::UIPstream::read(token& t)
         case token::tokenType::DIRECTIVE :
         {
             word val;
-            if (readStringFromBuffer(val))
+            if (readString(val))
             {
                 if (token::compound::isCompound(val))
                 {
@@ -258,7 +266,7 @@ Foam::Istream& Foam::UIPstream::read(token& t)
         case token::tokenType::VERBATIM :
         {
             string val;
-            if (readStringFromBuffer(val))
+            if (readString(val))
             {
                 t = std::move(val);
                 t.setType(token::tokenType(c));
@@ -335,8 +343,8 @@ Foam::Istream& Foam::UIPstream::read(token& t)
 
 Foam::Istream& Foam::UIPstream::read(char& c)
 {
-    c = externalBuf_[externalBufPosition_];
-    ++externalBufPosition_;
+    c = recvBuf_[recvBufPos_];
+    ++recvBufPos_;
     checkEof();
     return *this;
 }
@@ -344,13 +352,13 @@ Foam::Istream& Foam::UIPstream::read(char& c)
 
 Foam::Istream& Foam::UIPstream::read(word& str)
 {
-    return readStringFromBuffer(str);
+    return readString(str);
 }
 
 
 Foam::Istream& Foam::UIPstream::read(string& str)
 {
-    return readStringFromBuffer(str);
+    return readString(str);
 }
 
 
@@ -377,9 +385,14 @@ Foam::Istream& Foam::UIPstream::read(doubleScalar& val)
 
 Foam::Istream& Foam::UIPstream::read(char* data, std::streamsize count)
 {
-    beginRawRead();
-    readRaw(data, count);
-    endRawRead();
+    if (count)
+    {
+        // For count == 0, a no-op
+        // - see UOPstream::write(const char*, streamsize)
+        beginRawRead();
+        readRaw(data, count);
+        endRawRead();
+    }
 
     return *this;
 }
@@ -405,7 +418,9 @@ bool Foam::UIPstream::beginRawRead()
             << Foam::abort(FatalError);
     }
 
-    // Alignment = 8, as per read(const char*, streamsize)
+    // Align on word boundary (64-bit)
+    // - as per read(const char*, streamsize)
+    // The check for zero-size will have been done by the caller
     prepareBuffer(8);
 
     return true;
@@ -414,7 +429,7 @@ bool Foam::UIPstream::beginRawRead()
 
 void Foam::UIPstream::rewind()
 {
-    externalBufPosition_ = 0;
+    recvBufPos_ = 0;
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
index 9a6b8e7ae8..7b44640fc8 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
@@ -63,9 +63,9 @@ class UIPstream
 
         int fromProcNo_;
 
-        DynamicList<char>& externalBuf_;
+        DynamicList<char>& recvBuf_;
 
-        label& externalBufPosition_;
+        label& recvBufPos_;
 
         const int tag_;
 
@@ -78,45 +78,45 @@ class UIPstream
 
     // Private Member Functions
 
-        //- Check the bufferPosition against messageSize_ for EOF
+        //- Check buffer position against messageSize_ for EOF
         inline void checkEof();
 
-        //- Prepare transfer buffer by adjusting alignment
+        //- Prepare receive buffer by adjusting alignment
         inline void prepareBuffer(const size_t align);
 
-        //- Read a T from the transfer buffer
+        //- Read a T from the receive buffer
         template<class T>
         inline void readFromBuffer(T& val);
 
-        //- Read count bytes of data from the transfer buffer.
+        //- Read count bytes of data from the receive buffer.
         //  Prior data alignment is done by prepareBuffer
         inline void readFromBuffer(void* data, const size_t count);
 
-        //- Read string length and its content.
-        inline Istream& readStringFromBuffer(std::string& str);
+        //- Read string length and string content
+        inline Istream& readString(std::string& str);
 
 
 public:
 
     // Constructors
 
-        //- Construct given process index to read from and optional buffer size,
-        //- read format and IO version
+        //- Construct given process index to read from using the given
+        //- attached receive buffer, optional communication characteristics
+        //- and IO format
         UIPstream
         (
             const commsTypes commsType,
             const int fromProcNo,
-            DynamicList<char>& externalBuf,
-            label& externalBufPosition,
+            DynamicList<char>& receiveBuf,
+            label& receiveBufPosition,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm,
-            const bool clearAtEnd = false,   // destroy externalBuf if at end
-            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY,
-            IOstreamOption::versionNumber ver = IOstreamOption::currentVersion
+            const bool clearAtEnd = false,   // destroy receiveBuf if at end
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
         //- Construct given buffers
-        UIPstream(const int fromProcNo, PstreamBuffers&);
+        UIPstream(const int fromProcNo, PstreamBuffers& buffers);
 
 
     //- Destructor
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
index 5480f63a73..d65a2a5011 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
@@ -31,6 +31,25 @@ License
 #include "token.H"
 #include <cctype>
 
+// * * * * * * * * * * * * * * * Local Functions * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+
+// Return the position with word boundary alignment
+inline static label byteAlign(const label pos, const size_t align)
+{
+    return
+    (
+        (align > 1)
+      ? (align + ((pos - 1) & ~(align - 1)))
+      : pos
+    );
+}
+
+} // End namespace Foam
+
+
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
 inline void Foam::UOPstream::prepareBuffer
@@ -44,14 +63,8 @@ inline void Foam::UOPstream::prepareBuffer
         return;
     }
 
-    // The current output position
-    label pos = sendBuf_.size();
-
-    if (align > 1)
-    {
-        // Align output position. Pads sendBuf_.size() - oldPos characters.
-        pos = align + ((pos - 1) & ~(align - 1));
-    }
+    // Align for the next output position
+    const label pos = byteAlign(sendBuf_.size(), align);
 
     // Extend buffer (as required)
     sendBuf_.reserve(max(1000, label(pos + count)));
@@ -68,16 +81,6 @@ inline void Foam::UOPstream::writeToBuffer(const T& val)
 }
 
 
-inline void Foam::UOPstream::writeToBuffer(const char& c)
-{
-    if (!sendBuf_.capacity())
-    {
-        sendBuf_.setCapacity(1000);
-    }
-    sendBuf_.append(c);
-}
-
-
 inline void Foam::UOPstream::writeToBuffer
 (
     const void* data,
@@ -98,7 +101,7 @@ inline void Foam::UOPstream::writeToBuffer
     // Extend the addressable range for direct pointer access
     sendBuf_.resize(pos + count);
 
-    char* const __restrict__ buf = (sendBuf_.begin() + pos);
+    char* const __restrict__ buf = (sendBuf_.data() + pos);
     const char* const __restrict__ input = reinterpret_cast<const char*>(data);
 
     for (size_t i = 0; i < count; ++i)
@@ -108,11 +111,21 @@ inline void Foam::UOPstream::writeToBuffer
 }
 
 
-inline void Foam::UOPstream::writeStringToBuffer(const std::string& str)
+inline void Foam::UOPstream::putChar(const char c)
+{
+    if (!sendBuf_.capacity())
+    {
+        sendBuf_.setCapacity(1000);
+    }
+    sendBuf_.append(c);
+}
+
+
+inline void Foam::UOPstream::putString(const std::string& str)
 {
     const size_t len = str.size();
     writeToBuffer(len);
-    writeToBuffer(str.data(), len, 1);
+    writeToBuffer(str.data(), len, 1);  // no-op when len == 0
 }
 
 
@@ -126,12 +139,11 @@ Foam::UOPstream::UOPstream
     const int tag,
     const label comm,
     const bool sendAtDestruct,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     UPstream(commsType),
-    Ostream(fmt, ver),
+    Ostream(fmt, IOstreamOption::currentVersion),
     toProcNo_(toProcNo),
     sendBuf_(sendBuf),
     tag_(tag),
@@ -146,7 +158,7 @@ Foam::UOPstream::UOPstream
 Foam::UOPstream::UOPstream(const int toProcNo, PstreamBuffers& buffers)
 :
     UPstream(buffers.commsType_),
-    Ostream(buffers.format_, buffers.version_),
+    Ostream(buffers.format_, IOstreamOption::currentVersion),
     toProcNo_(toProcNo),
     sendBuf_(buffers.sendBuf_[toProcNo]),
     tag_(buffers.tag_),
@@ -196,8 +208,8 @@ bool Foam::UOPstream::write(const token& tok)
     {
         case token::tokenType::FLAG :
         {
-            writeToBuffer(char(token::tokenType::FLAG));
-            writeToBuffer(char(tok.flagToken()));
+            putChar(token::tokenType::FLAG);
+            putChar(tok.flagToken());
 
             return true;
         }
@@ -206,8 +218,8 @@ bool Foam::UOPstream::write(const token& tok)
         case token::tokenType::WORD :
         case token::tokenType::DIRECTIVE :
         {
-            writeToBuffer(char(tok.type()));
-            writeStringToBuffer(tok.wordToken());
+            putChar(tok.type());
+            putString(tok.wordToken());
 
             return true;
         }
@@ -218,8 +230,8 @@ bool Foam::UOPstream::write(const token& tok)
         case token::tokenType::VARIABLE :
         case token::tokenType::VERBATIM :
         {
-            writeToBuffer(char(tok.type()));
-            writeStringToBuffer(tok.stringToken());
+            putChar(tok.type());
+            putString(tok.stringToken());
 
             return true;
         }
@@ -236,7 +248,7 @@ Foam::Ostream& Foam::UOPstream::write(const char c)
 {
     if (!isspace(c))
     {
-        writeToBuffer(c);
+        putChar(c);
     }
 
     return *this;
@@ -262,8 +274,8 @@ Foam::Ostream& Foam::UOPstream::write(const char* str)
 
 Foam::Ostream& Foam::UOPstream::write(const word& str)
 {
-    writeToBuffer(char(token::tokenType::WORD));
-    writeStringToBuffer(str);
+    putChar(token::tokenType::WORD);
+    putString(str);
 
     return *this;
 }
@@ -271,8 +283,8 @@ Foam::Ostream& Foam::UOPstream::write(const word& str)
 
 Foam::Ostream& Foam::UOPstream::write(const string& str)
 {
-    writeToBuffer(char(token::tokenType::STRING));
-    writeStringToBuffer(str);
+    putChar(token::tokenType::STRING);
+    putString(str);
 
     return *this;
 }
@@ -286,13 +298,13 @@ Foam::Ostream& Foam::UOPstream::writeQuoted
 {
     if (quoted)
     {
-        writeToBuffer(char(token::tokenType::STRING));
+        putChar(token::tokenType::STRING);
     }
     else
     {
-        writeToBuffer(char(token::tokenType::WORD));
+        putChar(token::tokenType::WORD);
     }
-    writeStringToBuffer(str);
+    putString(str);
 
     return *this;
 }
@@ -300,7 +312,7 @@ Foam::Ostream& Foam::UOPstream::writeQuoted
 
 Foam::Ostream& Foam::UOPstream::write(const int32_t val)
 {
-    writeToBuffer(char(token::tokenType::LABEL));
+    putChar(token::tokenType::LABEL);
     writeToBuffer(val);
     return *this;
 }
@@ -308,7 +320,7 @@ Foam::Ostream& Foam::UOPstream::write(const int32_t val)
 
 Foam::Ostream& Foam::UOPstream::write(const int64_t val)
 {
-    writeToBuffer(char(token::tokenType::LABEL));
+    putChar(token::tokenType::LABEL);
     writeToBuffer(val);
     return *this;
 }
@@ -316,7 +328,7 @@ Foam::Ostream& Foam::UOPstream::write(const int64_t val)
 
 Foam::Ostream& Foam::UOPstream::write(const floatScalar val)
 {
-    writeToBuffer(char(token::tokenType::FLOAT));
+    putChar(token::tokenType::FLOAT);
     writeToBuffer(val);
     return *this;
 }
@@ -324,7 +336,7 @@ Foam::Ostream& Foam::UOPstream::write(const floatScalar val)
 
 Foam::Ostream& Foam::UOPstream::write(const doubleScalar val)
 {
-    writeToBuffer(char(token::tokenType::DOUBLE));
+    putChar(token::tokenType::DOUBLE);
     writeToBuffer(val);
     return *this;
 }
@@ -339,6 +351,7 @@ Foam::Ostream& Foam::UOPstream::write(const char* data, std::streamsize count)
             << Foam::abort(FatalError);
     }
 
+    // Align on word boundary (64-bit)
     writeToBuffer(data, count, 8);
 
     return *this;
@@ -370,7 +383,8 @@ bool Foam::UOPstream::beginRawWrite(std::streamsize count)
             << Foam::abort(FatalError);
     }
 
-    // Alignment = 8, as per write(const char*, streamsize)
+    // Align on word boundary (64-bit)
+    // - as per write(const char*, streamsize)
     prepareBuffer(count, 8);
 
     return true;
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
index 68393e97be..d4eefe305e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
@@ -75,18 +75,16 @@ class UOPstream
 
     // Private Member Functions
 
-        //- Prepare buffer for count bytes of output at specified alignment.
+        //- Prepare send buffer for count bytes of output,
+        //- with specified alignment.
         inline void prepareBuffer(const size_t count, const size_t align);
 
-        //- Write data to the transfer buffer
+        //- Generic write data to the send buffer, aligned by sizeof(T)
         template<class T>
         inline void writeToBuffer(const T& val);
 
-        //- Write a char to the transfer buffer
-        inline void writeToBuffer(const char& c);
-
-        //- Write count bytes of data to the transfer buffer
-        //  using align byte alignment
+        //- Write count bytes of data to the send buffer,
+        //- using align byte alignment
         inline void writeToBuffer
         (
             const void* data,
@@ -94,17 +92,21 @@ class UOPstream
             const size_t align
         );
 
-        //- Write string length and content.
+        //- Add a single char to the send buffer. No alignment needed
+        inline void putChar(const char c);
+
+        //- Write string length and string content.
         //  The content includes the trailing nul char.
-        inline void writeStringToBuffer(const std::string& str);
+        inline void putString(const std::string& str);
 
 
 public:
 
     // Constructors
 
-        //- Construct given process index to send to and optional buffer size,
-        //- write format and IO version
+        //- Construct given process index to write to using the given
+        //- attached send buffer, optional communication characteristics
+        //- and IO format
         UOPstream
         (
             const commsTypes commsType,
@@ -113,15 +115,14 @@ public:
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm,
             const bool sendAtDestruct = true,
-            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY,
-            IOstreamOption::versionNumber ver = IOstreamOption::currentVersion
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
         //- Construct given buffers
         UOPstream(const int toProcNo, PstreamBuffers& buffers);
 
 
-    //- Destructor
+    //- Destructor.
     ~UOPstream();
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index f3ff593222..b87f219426 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -106,7 +106,7 @@ Foam::label Foam::UPstream::allocateCommunicator
     label index;
     if (!freeComms_.empty())
     {
-        index = freeComms_.pop();
+        index = freeComms_.remove();  // LIFO pop
     }
     else
     {
@@ -114,10 +114,10 @@ Foam::label Foam::UPstream::allocateCommunicator
         index = parentCommunicator_.size();
 
         myProcNo_.append(-1);
-        procIDs_.append(List<int>(0));
+        procIDs_.append(List<int>());
         parentCommunicator_.append(-1);
-        linearCommunication_.append(List<commsStruct>(0));
-        treeCommunication_.append(List<commsStruct>(0));
+        linearCommunication_.append(List<commsStruct>());
+        treeCommunication_.append(List<commsStruct>());
     }
 
     if (debug)
@@ -186,7 +186,7 @@ void Foam::UPstream::freeCommunicator
     linearCommunication_[communicator].clear();
     treeCommunication_[communicator].clear();
 
-    freeComms_.push(communicator);
+    freeComms_.append(communicator);  // LIFO push
 }
 
 
@@ -365,14 +365,14 @@ bool Foam::UPstream::haveThreads_(false);
 int Foam::UPstream::msgType_(1);
 
 
-Foam::LIFOStack<Foam::label> Foam::UPstream::freeComms_;
-
 Foam::DynamicList<int> Foam::UPstream::myProcNo_(10);
 
 Foam::DynamicList<Foam::List<int>> Foam::UPstream::procIDs_(10);
 
 Foam::DynamicList<Foam::label> Foam::UPstream::parentCommunicator_(10);
 
+Foam::DynamicList<Foam::label> Foam::UPstream::freeComms_;
+
 Foam::wordList Foam::UPstream::allWorlds_(Foam::one{}, "");
 Foam::labelList Foam::UPstream::worldIDs_(Foam::one{}, 0);
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 32042e4cbc..63a00f9217 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -48,7 +48,6 @@ SourceFiles
 #include "string.H"
 #include "Enum.H"
 #include "ListOps.H"
-#include "LIFOStack.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -67,7 +66,7 @@ public:
     typedef IntRange<int> rangeType;
 
     //- Types of communications
-    enum class commsTypes
+    enum class commsTypes : char
     {
         blocking,
         scheduled,
@@ -83,7 +82,7 @@ public:
         //- Structure for communicating between processors
         class commsStruct
         {
-            // Private data
+            // Private Data
 
                 //- procID of above processor
                 label above_;
@@ -103,7 +102,7 @@ public:
 
             // Constructors
 
-                //- Construct null
+                //- Default construct. Above == -1
                 commsStruct();
 
                 //- Construct from components
@@ -128,33 +127,30 @@ public:
 
             // Member Functions
 
-                // Access
+                label above() const noexcept
+                {
+                    return above_;
+                }
 
-                    label above() const
-                    {
-                        return above_;
-                    }
+                const labelList& below() const noexcept
+                {
+                    return below_;
+                }
 
-                    const labelList& below() const
-                    {
-                        return below_;
-                    }
+                const labelList& allBelow() const noexcept
+                {
+                    return allBelow_;
+                }
 
-                    const labelList& allBelow() const
-                    {
-                        return allBelow_;
-                    }
-
-                    const labelList& allNotBelow() const
-                    {
-                        return allNotBelow_;
-                    }
+                const labelList& allNotBelow() const noexcept
+                {
+                    return allNotBelow_;
+                }
 
 
-            // Member operators
+            // Member Operators
 
                 bool operator==(const commsStruct&) const;
-
                 bool operator!=(const commsStruct&) const;
 
 
@@ -183,7 +179,7 @@ public:
 
 private:
 
-    // Private Data
+    // Private Static Data
 
         //- By default this is not a parallel run
         static bool parRun_;
@@ -203,9 +199,6 @@ private:
 
         // Communicator specific data
 
-        //- Free communicators
-        static LIFOStack<label> freeComms_;
-
         //- My processor number
         static DynamicList<int> myProcNo_;
 
@@ -215,6 +208,9 @@ private:
         //- Parent communicator
         static DynamicList<label> parentCommunicator_;
 
+        //- Free communicators
+        static DynamicList<label> freeComms_;
+
         //- Linear communication schedule
         static DynamicList<List<commsStruct>> linearCommunication_;
 
@@ -263,6 +259,7 @@ protected:
         //- Communications type of this stream
         commsTypes commsType_;
 
+
 public:
 
     // Declare name of the class and its debug switch
@@ -301,8 +298,8 @@ public:
 
     // Constructors
 
-        //- Construct given optional buffer size
-        UPstream(const commsTypes commsType)
+        //- Construct for given communication type
+        explicit UPstream(const commsTypes commsType)
         :
             commsType_(commsType)
         {}
@@ -356,7 +353,7 @@ public:
                 freeCommunicator(comm_);
             }
 
-            operator label() const
+            operator label() const noexcept
             {
                 return comm_;
             }
@@ -424,21 +421,22 @@ public:
 
         //- Set as parallel run on/off.
         //  \return the previous value
-        static bool parRun(const bool on)
+        static bool parRun(const bool on) noexcept
         {
             bool old(parRun_);
             parRun_ = on;
             return old;
         }
 
-        //- Test if this a parallel run, or allow modify access
-        static bool& parRun()
+        //- Test if this a parallel run
+        //  Modify access is deprecated
+        static bool& parRun() noexcept
         {
             return parRun_;
         }
 
         //- Have support for threads
-        static bool haveThreads()
+        static bool haveThreads() noexcept
         {
             return haveThreads_;
         }
@@ -482,13 +480,13 @@ public:
         // Worlds
 
             //- All worlds
-            static const wordList& allWorlds()
+            static const wordList& allWorlds() noexcept
             {
                 return allWorlds_;
             }
 
             //- worldID (index in allWorlds) of all processes
-            static const labelList& worldIDs()
+            static const labelList& worldIDs() noexcept
             {
                 return worldIDs_;
             }
@@ -539,24 +537,24 @@ public:
         }
 
         //- Message tag of standard messages
-        static int& msgType()
+        static int& msgType() noexcept
         {
             return msgType_;
         }
 
 
         //- Get the communications type of the stream
-        commsTypes commsType() const
+        commsTypes commsType() const noexcept
         {
             return commsType_;
         }
 
         //- Set the communications type of the stream
-        commsTypes commsType(const commsTypes ct)
+        commsTypes commsType(const commsTypes ct) noexcept
         {
-            commsTypes oldCommsType = commsType_;
+            commsTypes old(commsType_);
             commsType_ = ct;
-            return oldCommsType;
+            return old;
         }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
index c4a7aca4f5..b7cbe95b69 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -26,16 +27,15 @@ License
 \*---------------------------------------------------------------------------*/
 
 #include "UPstream.H"
-#include "boolList.H"
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 Foam::UPstream::commsStruct::commsStruct()
 :
     above_(-1),
-    below_(0),
-    allBelow_(0),
-    allNotBelow_(0)
+    below_(),
+    allBelow_(),
+    allNotBelow_()
 {}
 
 
diff --git a/src/Pstream/dummy/UIPread.C b/src/Pstream/dummy/UIPread.C
index 3663e35f59..f6cc67da04 100644
--- a/src/Pstream/dummy/UIPread.C
+++ b/src/Pstream/dummy/UIPread.C
@@ -37,20 +37,19 @@ Foam::UIPstream::UIPstream
 (
     const commsTypes commsType,
     const int fromProcNo,
-    DynamicList<char>& externalBuf,
-    label& externalBufPosition,
+    DynamicList<char>& receiveBuf,
+    label& receiveBufPosition,
     const int tag,
     const label comm,
     const bool clearAtEnd,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     UPstream(commsType),
-    Istream(fmt, ver),
+    Istream(fmt, IOstreamOption::currentVersion),
     fromProcNo_(fromProcNo),
-    externalBuf_(externalBuf),
-    externalBufPosition_(externalBufPosition),
+    recvBuf_(receiveBuf),
+    recvBufPos_(receiveBufPosition),
     tag_(tag),
     comm_(comm),
     clearAtEnd_(clearAtEnd),
@@ -63,10 +62,10 @@ Foam::UIPstream::UIPstream
 Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
 :
     UPstream(buffers.commsType_),
-    Istream(buffers.format_, buffers.version_),
+    Istream(buffers.format_, IOstreamOption::currentVersion),
     fromProcNo_(fromProcNo),
-    externalBuf_(buffers.recvBuf_[fromProcNo]),
-    externalBufPosition_(buffers.recvBufPos_[fromProcNo]),
+    recvBuf_(buffers.recvBuf_[fromProcNo]),
+    recvBufPos_(buffers.recvBufPos_[fromProcNo]),
     tag_(buffers.tag_),
     comm_(buffers.comm_),
     clearAtEnd_(true),
diff --git a/src/Pstream/mpi/UIPread.C b/src/Pstream/mpi/UIPread.C
index 6b04ff48b6..bd8b318899 100644
--- a/src/Pstream/mpi/UIPread.C
+++ b/src/Pstream/mpi/UIPread.C
@@ -42,20 +42,19 @@ Foam::UIPstream::UIPstream
 (
     const commsTypes commsType,
     const int fromProcNo,
-    DynamicList<char>& externalBuf,
-    label& externalBufPosition,
+    DynamicList<char>& receiveBuf,
+    label& receiveBufPosition,
     const int tag,
     const label comm,
     const bool clearAtEnd,
-    IOstreamOption::streamFormat fmt,
-    IOstreamOption::versionNumber ver
+    IOstreamOption::streamFormat fmt
 )
 :
     UPstream(commsType),
-    Istream(fmt, ver),
+    Istream(fmt, IOstreamOption::currentVersion),
     fromProcNo_(fromProcNo),
-    externalBuf_(externalBuf),
-    externalBufPosition_(externalBufPosition),
+    recvBuf_(receiveBuf),
+    recvBufPos_(receiveBufPosition),
     tag_(tag),
     comm_(comm),
     clearAtEnd_(clearAtEnd),
@@ -66,29 +65,26 @@ Foam::UIPstream::UIPstream
 
     if (commsType == commsTypes::nonBlocking)
     {
-        // Message is already received into externalBuf
+        // Message is already received into buffer
     }
     else
     {
-        MPI_Status status;
-
-        label wantedSize = externalBuf_.capacity();
-
         if (debug)
         {
-            Pout<< "UIPstream::UIPstream : read from:" << fromProcNo
-                << " tag:" << tag << " comm:" << comm_
-                << " wanted size:" << wantedSize
+            Pout<< "UIPstream::UIPstream :"
+                << " read from:" << fromProcNo
+                << " tag:" << tag_ << " comm:" << comm_
+                << " wanted size:" << recvBuf_.capacity()
                 << Foam::endl;
         }
 
-
-        // If the buffer size is not specified, probe the incoming message
-        // and set it
-        if (!wantedSize)
+        // No buffer size allocated/specified - probe size of incoming message
+        if (!recvBuf_.capacity())
         {
             profilingPstream::beginTiming();
 
+            MPI_Status status;
+
             MPI_Probe
             (
                 fromProcNo_,
@@ -101,13 +97,12 @@ Foam::UIPstream::UIPstream
             // Assume these are from gathers ...
             profilingPstream::addGatherTime();
 
-            externalBuf_.setCapacity(messageSize_);
-            wantedSize = messageSize_;
+            recvBuf_.resize(messageSize_);
 
             if (debug)
             {
-                Pout<< "UIPstream::UIPstream : probed size:" << wantedSize
-                    << Foam::endl;
+                Pout<< "UIPstream::UIPstream : probed size:"
+                    << messageSize_ << Foam::endl;
             }
         }
 
@@ -115,14 +110,14 @@ Foam::UIPstream::UIPstream
         (
             commsType,
             fromProcNo_,
-            externalBuf_.data(),
-            wantedSize,
+            recvBuf_.data(),
+            recvBuf_.capacity(),
             tag_,
             comm_
         );
 
         // Set addressed size. Leave actual allocated memory intact.
-        externalBuf_.setSize(messageSize_);
+        recvBuf_.resize(messageSize_);
 
         if (!messageSize_)
         {
@@ -135,10 +130,10 @@ Foam::UIPstream::UIPstream
 Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
 :
     UPstream(buffers.commsType_),
-    Istream(buffers.format_, buffers.version_),
+    Istream(buffers.format_, IOstreamOption::currentVersion),
     fromProcNo_(fromProcNo),
-    externalBuf_(buffers.recvBuf_[fromProcNo]),
-    externalBufPosition_(buffers.recvBufPos_[fromProcNo]),
+    recvBuf_(buffers.recvBuf_[fromProcNo]),
+    recvBufPos_(buffers.recvBufPos_[fromProcNo]),
     tag_(buffers.tag_),
     comm_(buffers.comm_),
     clearAtEnd_(true),
@@ -162,8 +157,8 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
 
     if (commsType() == commsTypes::nonBlocking)
     {
-        // Message is already received into externalBuf
-        messageSize_ = buffers.recvBuf_[fromProcNo].size();
+        // Message is already received into buffer
+        messageSize_ = recvBuf_.size();
 
         if (debug)
         {
@@ -176,25 +171,22 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
     }
     else
     {
-        MPI_Status status;
-
-        label wantedSize = externalBuf_.capacity();
-
         if (debug)
         {
             Pout<< "UIPstream::UIPstream PstreamBuffers :"
                 << " read from:" << fromProcNo
                 << " tag:" << tag_ << " comm:" << comm_
-                << " wanted size:" << wantedSize
+                << " wanted size:" << recvBuf_.capacity()
                 << Foam::endl;
         }
 
-        // If the buffer size is not specified, probe the incoming message
-        // and set it
-        if (!wantedSize)
+        // No buffer size allocated/specified - probe size of incoming message
+        if (!recvBuf_.capacity())
         {
             profilingPstream::beginTiming();
 
+            MPI_Status status;
+
             MPI_Probe
             (
                 fromProcNo_,
@@ -207,13 +199,12 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
             // Assume these are from gathers ...
             profilingPstream::addGatherTime();
 
-            externalBuf_.setCapacity(messageSize_);
-            wantedSize = messageSize_;
+            recvBuf_.resize(messageSize_);
 
             if (debug)
             {
                 Pout<< "UIPstream::UIPstream PstreamBuffers : probed size:"
-                    << wantedSize << Foam::endl;
+                    << messageSize_ << Foam::endl;
             }
         }
 
@@ -221,14 +212,14 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
         (
             commsType(),
             fromProcNo_,
-            externalBuf_.data(),
-            wantedSize,
+            recvBuf_.data(),
+            recvBuf_.capacity(),
             tag_,
             comm_
         );
 
         // Set addressed size. Leave actual allocated memory intact.
-        externalBuf_.setSize(messageSize_);
+        recvBuf_.resize(messageSize_);
 
         if (!messageSize_)
         {
@@ -271,7 +262,11 @@ Foam::label Foam::UIPstream::read
 
     profilingPstream::beginTiming();
 
-    if (commsType == commsTypes::blocking || commsType == commsTypes::scheduled)
+    if
+    (
+        commsType == commsTypes::blocking
+     || commsType == commsTypes::scheduled
+    )
     {
         MPI_Status status;
 
diff --git a/src/Pstream/mpi/UOPwrite.C b/src/Pstream/mpi/UOPwrite.C
index a4081d2a13..f59fba924d 100644
--- a/src/Pstream/mpi/UOPwrite.C
+++ b/src/Pstream/mpi/UOPwrite.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -81,9 +81,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_BYTE,
-            toProcNo,   //procID(toProcNo),
+            toProcNo,
             tag,
-            PstreamGlobals::MPICommunicators_[communicator] //MPI_COMM_WORLD
+            PstreamGlobals::MPICommunicators_[communicator]
         );
 
         // Assume these are from scatters ...
@@ -104,9 +104,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_BYTE,
-            toProcNo,   //procID(toProcNo),
+            toProcNo,
             tag,
-            PstreamGlobals::MPICommunicators_[communicator] //MPI_COMM_WORLD
+            PstreamGlobals::MPICommunicators_[communicator]
         );
 
         // Assume these are from scatters ...
@@ -129,9 +129,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_BYTE,
-            toProcNo,   //procID(toProcNo),
+            toProcNo,
             tag,
-            PstreamGlobals::MPICommunicators_[communicator],//MPI_COMM_WORLD,
+            PstreamGlobals::MPICommunicators_[communicator],
             &request
         );