I'm totally spitballing here, but soldiers were known to hammer out their own poker chips out of buttons/bullets and anything else they could get their hands on when supplies ran low. Perhaps this is an example of that?
I'm stumped, but I like the theory that it might be a front cap over a cannister round. I don't know if they were made of lead, but that might account for round indentations. On the other hand, they might have been made with a ball peen hammer.