Done a bit of googling, and it seems they are not that uncommon, or a least weren't at one time.
From the good old Wiki, on Hammers......
"As the impact between steel hammer heads and the objects being hit can, and does, create sparks, which in some industries such as underground coal mining with methane gas, or in other hazardous environments containing flammable gases and vapours, can be dangerous and risk igniting the gases. In these environments, a variety of non-sparking metal tools are used, being principally, aluminium or beryllium copper-headed hammers.[3]"
Think someone mentioned they could be beryllium copper already.
Wonder what that little hole is for? Must be there for a reason.