This is a handle designed to fit the tang of an Estwing hammer. The one I designed it for was their drywall hammer. I have the leather handled claw hammer as well, and that one is a bit shorter, so some scaling may be required to fit to a specific model.
The tang dimensions mine had, and this design fits, are 185mm length, 22mm wide at the bolster, 21mm wide at the far end of the tang. There was a hole, 1mm off centre, 9mm from the end of the tang, I included a way to affix a bolt, which I didn't bother doing with mine as the fit was plenty tight.