I see the underlying outline and it does look like a D mint mark. Wait too see what some of the RPM folks have to say
I'm not an RPM expert, but it looks to me like the S is filled a little, not that there was a D under there.
If it was a omm the D & S would be the same size. What looks to be a D in the lower part of the S is a die chip or MD. There are 6 RPM'S none match the op image. It is possible for a different die state, you have to post the full image of the obv. For other pup's to be seen.
Golly that's so funny it looks like a D. But it ain't, guys. The Ds on the 1953s weren't shaped like that, for one. Look at how narrow and upright that "D" is. The Ds on the 1953s were wide. Think of the 1864 two-cent and those Ds in GOD marking the two varieties. The Ds on the 1953s were more like the Small Motto Ds. Now that I think of it, I don't think any of the Ds on the 1950s are upright, like this one. If Coop drops in, he's seen more RPMs than anybody has a right to, he ought to tell.