Strange cladding on a dime

Found this dime today during an error hunt. The clad layer protrudes approximately 1mm from the copper layer, but both layers are still reeded. What could possibly cause this?

 

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My guess would be acid. It looks like the surface of the coin has a bit of pitting which might support this, but if you had an acid that reacts with copper then it would eat away at the copper core.

 

Also remember that the copper core is w-a-a-y softer than that copper-nickel cladding.

 

Oh well, I'm really striking out lately! Thanks for the assist.

 

I have found halves like this and have been told acid.....I still kept them (3), as they are neat.