Coinsandcanada.com

I found 2 PLsets of 1976 with a strange penny inside. The penny have ghostly form around letters and date. I want to identify the origin of this error. I know that another member have a penny of 1963 like this one but from circulation roll.
It's look like mortar set but without any spots.
Here is some pictures:






Thank you for your help!

Could it be from over-zealous polishing of dies? I have 1949 five cent coins which are similar in that the lines follow the writing or image and almost look like doubling where the polishing is very close to the details.

Could be but this penny come from PLSET and still into envelope (cello). I was thinking that kind of dies to strike PL coins are brand new and not repolish.