I think the filter history is a bit confusing: I have a registered copy of IT2.14p2 (it will show up as p2 in OpenMPT/Schism), but it already has filters, and its UPDATE.TXT file (dated 21.12.1997) already mentions IT2.14p3 and the IT2.15 sample compression. Maybe it's really a p3 version but it was forgotten to increment the version number on some registered builds.
How do you tell which version you have? I find no way in the program to identify it. "IT -?" gives a help screen which prints "Impulse Tracker, Copyright (C) 1995-1999 Jeffrey Lim, Registered to [my name]", but no version number. The date of my IT.EXE is 17.06.99, size is 95930 bytes. Does not seem to match any of the ones in the link you supplied. But I do notice that 2.14p5 at Modland is more than 400 kb, so I guess the other ones (including mine) are compressed.
My FILTERS.TXT is dated 21.12.97, which coincides with the 2.14p3 "Merry XMas guys!" release. I don't have any of the network-related files from the 2.14p5 release. I do remember fiddling with the DirectX-stuff in 2.14p4, but in the end I just ditched it and used pure DOS mode for IT.
I came across a module in my collection called 303.it, which I also remember having back in the day. I believe this was shipped with one of the patches as a showcase of the filter capabilities in IT. Foobar reports this modules as 2.14p2.