I believe nova is correct. They had uses other than tool checks also.
They would hang one on an ore car that they loaded, and would hang
one on a board before they went down in the mine so someone would
always know who was down in the mine. Some had the mines name
embossed on them, and some were plain with just the number. I've
found about a hundred of them.
And if you're in an area where you find these, it's a good possibility
that you'll find other interesting items.
Medals
Tokens
And coins