I'd bet that the number on the PST (as has been said already) will either be the US Patent number, or the registered TM number, both of which have been stamped on PSTs over the years at various times.
If the tool has no datecodes in the handles, that would indicate that it was made prior to 1992, but there are other characteristics that can help us to narrow it down a bit.
This thread tells you loads about dating your PST, but if you post some pictures on here, I'm sure people will help you if they can.