Hmmm - how can I find (with an editor) the "full QCAD version number and build information saved in the DXF file"? My new drawings are created on Win with 3.12.4.0 but my editor can't find any Information regarding the QCAD Version. I was expecting the info on the top of the file but there is nothing what tells me in plain text about the QCAD version.

It's stored as a DXF XRecord, not a comment.
Look for a section like this:

  3
QCADVersion
350
12DE

Then look for the XRecord handle (here 12DE):

  0
XRECORD
  5
12DE
102
{ACAD_REACTORS
330
12DD
102
}
330
12DD
100
AcDbXrecord
280
     1
1000
3.7.2

The last line of this particular XRecord is the QCAD version string.

Ubuntu 19.10; QCAD 3.24.2

The QCAD version number will only be inserted in a dxf file the SECOND time you save the file in the SAME QCAD session.

Simple test: 1) start QCAD with no file; 2) save the untitled file as test.dxf (do not close QCAD); 3) grep test.dxf for QCADVersion -- it will not be found; 4) save the file a second time in QCAD; 5) grep test.dxf for QCADVersion -- this time it will be found!

Someone could create a file and only save it one time each time they started QCAD, and the file will never contain the QCAD version. Also, when creating a library item using Block|Create Library Item, the QCAD version will never be inserted.

I cannot reproduce this. Is the QCAD Professional or the free QCAD Community Edition (dxflib)?

I am using the Community Edition (999 comment in dxf file is dxflib 3.17.0.0). I have repeated the simple test many times, always with the result I documented. I just ran grep on a library of 45 library items I have created over the past several weeks, and none of the dxf files contain "QCADVersion". What behavior do you observe? Does the QCAD version appear in the Untitled layout the first time you save it? I am willing to perform more tests if you give me some direction. My background: retired Senior Principal Software Engineer for a major U.S. company. 80% of my effort over a 40 year career was developing Windows-based engineering tools and automated testing tools for our software developers and customers. Thanks.

Today I installed QCAD 3.24.2 on a Windows 10 PC. I deleted the DLLs for the Pro Edition, making it a Community Edition. I then repeated my simple test with exactly the same result as I have previously documented. I closed QCAD and restored the DLLs, then restarted QCAD and it came up as the Pro Edition. I then repeated my simple test and the dxf file contained QCADVersion after the first save. Apparently this is only an issue with the Community Edition. I hope this helps.