If it were a RPM, the upper loop of the mint mark would also have the same spread: It is the zinc under the plating that is raising. This is very common in the 80's. Note mostly on the SE corner of the mint mark. The punching of the mint mark leaves a fin that splits the plating and exposes the zinc to air which starts the seteriation. The problem was removed after 1990 because the mint mark was part of the design and no longer punched.
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