Looks like some kind of contaminant that leached outwards onto/into the metal. I'm sure there is no way to remove it without ruining the proof surface. That's too bad as it looks to be a very nice Proof Franklin otherwise.
to the CCF Whatever it is , it started out as the dark area in the middle of the spot and grew from there. QC was not even a thing back then. And as stated above, it's permanent.
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