Is that area recessed or not? If so, it would probably be some zinc rot, but if not, it may be a plating error. But it's probably part of the weird patchy toning on the coin.
Recessed. It looks like it was cut out of the surface. Can zinc rot cause just that cut out section to just come off?
Its tough to tell with the photos, does the area seem to be plated at all? Is there zinc exposed? I'm no expert but I'm thinking it could possibly be some sort of strike through. I'm sure someone more knowledgeable will come along and figure it out. I'm not sure but there definitely doesn't appear to be displaced metal.
Hi Imho, it could have been a zinc bubble and someone popped it or cut it out. As for the discoloration it is normal from oils. Circulation wear. Check out this website...scroll down a bit.. https://sciencing.com/do-pennies-change-color-5169366.html
