1938-D Buffalo OMM-4?

[BankNote1]

I just picked up, from the mail, a 1938-D Buffalo Nickel that I thought was a 1938-D/S RPM. The coin is in a ANACS holder but instead of having 1938-D/S or etc. on the holder, the label says 1938-D OMM-4?

I guess what is the difference between RPM and OMM?

If I sent this coin to NGC to cross and I think it will cross, what would NGC put on the holder? I have never seen an NGC holder with OMM- anything!?!?

Yes, I know I am dumb! I also know that I should of figured out what OMM was before I purchased this coin. But This Buffalo Nickel is a super MS66 and looks dripping wet!!!

 

I did look at the mintmark and it doesn't look like a D/D or D/S but it is a D over something but I can't tell what it is.

Thanks in advance for any help.