It looks to me like something may have bumped it pushing up that part that everyone is seeing as part of an S. Some closeups of the D from different angles ought to show if that's what it is. I've seen mint marks hit before and the metal displaced so I know that's plausible.
This is only applies to cents minted after '90 or '91. Previous to that, each branch mint hand punched their own dies. There was no reason for Denver to have had an S punch. Hommer had the answer. A D that took a hit.
Well, here are more pictureS from different angles. Not the best, but the best I can get with a cell phone!
That just screams "there's an S in here!" to me... I think it's a filled S that happens to look like a D. Whatever it is, it's a really cool coin and definitely a keeper.
Since 1990 the mintmarks have been in the master hub. Until 1996 Philadelphia produced all of the dies used there and at the branch mints. All mintmarks were in the dies before they were shipped out, the branch mints did not put the mintmarks into the dies themselves. The Denver die shop opened in 1996 and it produces all of the dies used there, but Philadelphia still produces the master hubs sent to the Denver shop. And Philadelphia still makes all the dies for the S and W branch mints. The OP coin looks to me like a hit on the lower left that pushed some metal up to look like the center of an S.
Reviving an old thread now that I can get close up pictures with a scope, and I ran across it after put away. The only thing is, I'm still now sure what it is. I tried is from different angles because of lighting, and now I see a notch in the lower left.