I have a 1984 penny it looks like a S on top of mm and maybe a D underneath. My son a picture I hope it helps. I know this sounds weird but in that picture it looks like the image of lincoln's chin lol. Any help from the pros I would be grateful for. Have a great day thanks.
Plating blister, if I had to guess. These post-1982 zinc cents are terrible in terms of quality. In short, gas expands between the core and the poorly bonded plating.
What year is your coin? What year did the mint switch to crappy zinc coated in copper? Yep, you've got one of those, but good eye and I too would have taken a much closer look at it, then ask here, so good job.
