I'm no expert, but I do like to take a shot at these...so...
I see a lot of bumps on the coin and it makes me think that the planchet was less than perfect and over the decades the impurities in the copper have out-gassed causing the bumps and that big bubble. I'm not sure that is possible, but I have seen art and jewelry-making websites where the artists complain about those sort of bumps and they are blaming it on impurities out-gassing.
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