The hammer head is the round plastic part at the end of the hammer that actually hits the chime.
All of the hammer heads are identical.
Unlike almost all of the other parts, the hammer heads should be printed with 100% infill. This uses much more of the plastic filament, but it adds weight to the hammer and makes the chime ring more clearly when struck.
Once printed, you may need to clean out the hole where the shaft will go using an 8 mm drill bit or a thin file. There is no need to make the hole absolutely perfect, so long as the head fits onto the shaft. In fact, a little roughness can help stop the hammer head from spinning on the shaft even when the end screw is not fully tightened.