Here is a mint set 71S LMC still in cello with a really dirty face. It reminds me of a coin that has been stepped on with dirty sneakers. Maybe a mint worker dropped it, stepped on it, and put it back to be packed but are there any other options explaining this look? I'll wait for some responses before breaking it out of the wrapper.
It's most likely a storage problem. The dark areas were in contact with the packaging. Some coins tone nice in the original packaging. But a lot don't. I found this look when opening some 70 U.C. sets. Just about every coin looks this way.
