@molly, first welcome to CCF. Second, I agree that this isn't a mint error, but rather the degradation of the zinc core beginning. It is odd how these blisters look a little bit like capital letter Ds.
"If you climb a good tree, you get a push." -----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed." -----King Adz