Looks like a "toned" waterspot. These newest zinkins from 2010 on suffer from reside left from cleaning, most of the time is a white spot, but eventually stains darker. Then again, just a stain? Or a beginning zinc zit?
I think I see what you are talking about. I agree with Crazy in that it appears to just be a stain or zinc rot (not a mint error). But better photos might help.
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