Used in engineering machine shops and the like to tap workpieces down into machine vices, whack things into place without damaging them, and so on. Thy can be had with two copper faces, or two rawhide faces ('bacon-rind hammer'), or one of each. They're made in several sizes.
The inserts can be bought separately for the user to fit (usually when the factory-fitted one wears out), but it's not an easy job because they're inserted into the cavity and the malleable iron of the head crimped round them to hold them in. Without the proper press tooling, that's well-nigh impossible. More pragmatic to just buy a new hammer if you really need one.
yup, buffalo hide, - I have refilled but maybe was lucky that the replacement hide was old and dry (shrunk? but must be basically the right size) -- I chamfered the end with a rasp, cleaned and lubed the socket and then pushed it in by closing a vice on it - didn't go all the way, treated with linseed oil and after a couple of years' whacking, it's further in.
alternative - make hardwood ends for it, giving a very useful wooden mallet with a bit of weight behind it - easy to replace striking faces.
Usually once side is copper and the other side is rawhide.
I picked one up with the rawhide missing but copper side never used. Since I already had a rawhide mallet I put lead in it instead and it's been fantastic, I can safely whack things like drill bits in the lathe without damaging them.