These boards are an easy "single circuit board" solution to managing multiline "1A2"
business phones popular in the 1950's - 1980's in the United States and other countries,
collectively called "key systems".
The current version of this board is REV-J5.
Board revisions are, in progression of new-to-old:
H / H1,
You can see the entire revision history
which describes in great detail what's different in each board revision.
If you're interested in these boards,
with the email Subject: "1A2 KSU Inquiry" and I can supply ordering info for
fully assembled/tested boards. Try to include info on which phone models you expect
to use with the board, and any other details/questions you can think of.