1990D LMC - incomplete/mishap plating or env dmg?

This is a weird one. With all the black gunk on it I just figured that there was some reaction that dissolved the copper plating, but then I saw that the gunk was still on the copper too, and that still seems somewhat intact.

I searched and found an incomplete plating error on error-ref and was wondering if something may have gone wrong with the plating bath...
http://www.error-ref.com/incomplete_plating/

I'm still leaning towards some sort of environmental damage, though. There's spots on the memorial that still have bits of copper on it which seems to imply the copper was rubbed off.

Thoughts on this?

 

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Its a zilincon after all!!!!,the plating was exposed and its starting to show its inner zinc.

 

Hah, probably right. I'm just curious what wore the plating off. I think it happened prior to the black gunk (grease?) since the gunk is on both the copper and on the zinc.

Maybe someone took a buffing tool to it or something.

 

This looks like the plating was removed then the coin corroded.