Found this in a roll recently. Probably a rotated D over D (or a post mint gouge in the mint mark), although I admit my eyes about popped out of my head since at first glance it looks like an S over D.
It looks like the part of the die that should punch the center of the "D" was broken off. Then the mint mark was dented after minting.
The 79 D has a similar looking D on some coins. I would think that it's been hit at some point then over time riding in ones pocket smoothed it over. Rhubarb