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.