A plug nickel is a nickel sized blank piece of metal fashioned from electrical junction boxes, usually. When one goes to connect one box to another a piece of round pre-punched metal is removed. These are about the size of a nickel and were often used in old vending machines and payphones. They required some smoothing out to make them round enough to work.

I've found gloves, bungee cords, bike taillights, 20.00 bill and about 20.00 more over the years. Oh yeah, on my way home from work one night I found a herd of donkeys on the road...about 30 of them. It was crazy. Took me about 2 months to find out where they'd come from. It was a 'Gentleman's Farm' about 2 miles away.