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At first, I thought it could be improper alloy but upon closer examination, this does not appear to be the case.
What causes you to think this now? If improper alloy is 'not' the case, then it is superficial discoloration, in which case, what would it matter what the cause was?
A common date in circulated condition, I might take an X-acto knife to the edge to see what color the coin metal really is.