I have a couple of old plierenches of the conventional type, but never saw this variation. I think Yadda is right, though, and these were probably for snaps or eyelets or both. Possibly punching leather or fabric, too. There were other similar tools with similar interchangeable fittings. The plierench, while a nifty idea, doesn't amplify power and I don't think it would make a very good metal punch. One other far out possibility: I once at a tag sale bought a very odd set of locking pliers, with another one of those "what the **** could this be for" jaws. They were for attaching ear tags to cows!