It's old adhesive attached to the Cent. The image of other cents were left on the dried glue. The backwards lettering and images is evidence of this. Dip it in acetone and it will come off. Not a mint error.
So this was etched down with some sort of chemical. Unfortunately not an error. The best way to learn about errors is to read up on them. I’ve found the website www.error-ref.com to be extremely helpful.
