Agree with a plating blister. I wonder if it was grown bigger by environment, maybe it started smaller but grew oxide and expanded and hasn't popped yet like most that we see.
Hello Mark! Thank you for posting pics and inquiring about this! I agree with the guys that these look like plating blisters, both the large one and the one next to Lincoln's head. Another thing it could be is zinc rot that is pushing up from the zinc core. Here's the definition from our glossary for Zinc Rot and if you click on the link below it, there's a photo example.
Zinc Rot: Zinc corrodes easily when exposed to the environment. When the plating is split on a copper-plated zinc cent, the zinc will often corrode under the plating, and push up on it creating a bigger and bigger fissure. Due to the strength of many of the hand-punched mint marks on pre-1990 business strike cent dies, this is a very common place on Lincoln cents for the plating to split and for this corrosion to take place.
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