The fill in the mint mark appears to be mintmark punch damage, chip out or deterrioration. If a RPM, there would be evidence of the rest of the mm, since this is not a well worn coin. Here is a 1970 S large date with RPM #2. You can see the second raised ( rather than flattened) beneath and merging at the serifs slightly. The top reddish line is pointing out a diagnostic marker line. Jim