I'm not sure what you're seeing... the single anomaly I see on the mintmark looks as if it may have been caused by a well-placed hit, or some other form of damage, at least from the less than ideal photo. I am also not aware of any known OMM for this date/mint.
I agree - after looking at the pics a little further it seems the so called S mint mark goes over the D mint mark, if D was over then S would be under it..