Mint-Mark placement does match either of the two D/S . http://varietyvista.com/02a LC RPMs Vol 1/OMMs 1951D.htm
I think Peter should institute a "Mail-In" program for people who can't take good close-up images. Chris
Maybe I'm just too "quality snobbish" for my own good, (Hey, what you mean 'maybe', pal?), but when I see that crud at the bottom, I INSTANTLY don't care about what ELSE it is.
I think you should call it a 1951-D 1/S! Looks clear to me, but I'm looking with 75-year-old eyeballs so you can take that for what it's worth.
Photos are too dark to be sure but it looks like something is there. Also looks like a 1 over an S too.