I found one of those recently, too, and all believed it was just copper plated. The solid copper color on the edge helps confirm that. There's also a treatment that brings out the copper color on the nickel cladding. If it was a wrong planchet error the weight should be off.
Here's the one I found while box hunting.


"I like money. I can't believe you like money, too. We should hang out."
Frito
Member CONECA, Early American Coppers, NETCC