That's what I thought originally but I took a second look before I put it in the return bin and I'm not entirely sure it's not a grease error. I need a better magnifier to tell for sure
Not a grease error, just a badly worn, damaged nickel. A stronger (better?) magnifier won’t show you anything that your eyes did at first. Trust your gut instincts.
Just a worn NICKEL in my opinion. Sorry, couldn't resist being a spelling policeman. The mix up is probably one of the most common errors made.
I would think some grease fill. There is other weakness and the mintmark is close enough to the rim it would have provided enough protection from wear that the rim would pretty much have to be gone for the mintmark to be worn away.