It is an RPM, but not FS-504. FS-504 is a san serif/trumpet tail S. It's 2 different styles of S mint mark (MMS-007/MMS-004). I wish I had time to try to match it up. It isn't any on Variety Vista, but Wexler or Coppercoins may have a match.
It could be a new one. I'm not seeing a match for it on coppercoins or Variety Vista. Wexler has one that looks close: 1947-S 1¢ WRPM-005 http://doubleddie.com/1504414.html Can't see if your coin has this marker or not? WRPM-019 also looks interesting.