I found this today and thought it was a bus token.
When I cleaned it off, there was no printing on it.
Slick on both sides.
I then thought it was the end piece of a fuse link cartridge but the slit is curved.
Any guesses?
I think Free Bird Tim could be on the right track. In the older days, electrical boxes and panels had plugs like that, where you put a flat edged screw driver in the slot and twisted it out. These came before the "knockout" type that he has pictured. My dad was an electrical contractor for 30 years, and I remember seeing those boxes with the slotted discs.