Yes, it is an RPM. Try comparing it to 1958D-1MM-011, 1958D-1MM-013, and 1958D-1MM-030. Let us know if you can match any markers. Congrats!
I am on a roll. Honestly, I could not have done it without that grab bag for initial in hands examples of things to look for. Again, no matter what others may think, it was $ well spent, just like a numismatic book.