Both the ancient Romans and Greeks would bury or burn their dead with a coin in the corpse’s mouth. They believed that the coin was needed to pay Charon, the ferryman who carried the recently deceased across the river Styx. The ancients also would put a tasty treat (something such as a piece of cake) in the hand of the stiff to give to Cerberus/Kerberos (Roman/Greek), the three-headed dog who welcomed the dead on the other bank of the river. Anyway, probably not what you are looking for but that’s the only thing I’ve ever heard about coins in mouths. Also, I have both a 10 kopek and a 20 kopek piece and neither have holes in them. Perhaps the pre-revolution Russian coins did but I don’t know.