If voter suppression is not yet in play, I vote no to a stain.
The highest magnified view of the anomaly the posting offers suggests to my mind that the composition is metal particles drawn from the area from 12 o'clock around to 6 o'clock on the anomaly. I see a large cavity to the right of the anomaly probably caused by the needed donation.
My guess is it's a phenomenon of either internal or external electrolysis -- impurities in the alloy, perhaps or improper social distancing.
Kevin