What is perplexing to me is the area south of the main mintmark.
I was determined to get a decent photo of that area. The Magnification is quite large, but my point is showing what could be the arch of either a "D" or an "S".
This coin was also shot at a slight angle, less than 30 degrees with the bottom of the cent elevated.
Have a Look:
Now to potentially debunk the possibility of a D/D/S, I changed the lighting and shot this coin at a different angle. In this photo the coin is NOT elevated, and lays flat. In this shot I tried to focus on something in the center of the main mint mark:
This one shot with the bottom of the coin facing the lighting, and the top of the coin elevated about 30 degrees may help the case that the portion south is looking more like a "D". But I'm not positive of that . If my eyes are NOT playing tricks on me, I can see a crossbar in the middle of the "D", which might line up in size, if they used the same mint mark punch:
One more close up of the "D", which shows impressions on the Arch of the "D" :
Full Obverse:
Full Reverse:
In the end, its simply a 1953 D/D RPM-10. The original owner probably seen something in a loupe, with the lighting just right and added the comment for further research.