To me it looks like a back of the finger knuckle smudge.
I know from experience. Was stupid and in a hurry and thought I was clever pressing a proof LMC into my album with my knuckle. Left a mark just like that. If that is what it is and it's fresh and hasn't etched the coins surface you might be able to remove it with an acetone soak.
I know from experience. Was stupid and in a hurry and thought I was clever pressing a proof LMC into my album with my knuckle. Left a mark just like that. If that is what it is and it's fresh and hasn't etched the coins surface you might be able to remove it with an acetone soak.

























