That is what it is. Fingerprints on a cent, and the salt eats into the plating or into the copper on the cent. They are permanent. They turn the original surface a brown color. Thus we have the brown coinage when the prints totally take over.
I can just imagine someone--having just consumed a couple slices of pepperoni pizza--finger the coin back in 2006 when it was fresh. Yup, scooped-up the change from the server's bill tray and dumped it into a pocket.
Fingers smeared with animal based cooking fats are probably the most injurious to staining coins. I have even seen stainless steel corroded by water, with a film of rancid fat floating on it.
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