IMO due to its age, I'm thinking zinc rot, a small hole has allowed air to enter under the copper plating causing the zinc core to corrode and the corrosion has traveled in a direction where the plating was not firmly attached to the zinc.
Edited: Do you have a scale? Have you weigh it? Wouldn't it be nice for the OP if its weight was 3.1...

Edited by Greasy Fingers
07/14/2021 11:22 pm