The pins do fit the jaws of a vise . It’s quite possible the it’s for holding something . He was a very good friend of mine and I’m sure he would’ve told me if he was engraving revolvers he only did soft metals.
Good plan. The guy who I got all these tools off sadly passed away but I remember seeing this thing on his bench quite often. So that’s why I’m assuming it’s something to do with engraving.