Hi Ye Old Collector,
(((lack of knowledge on US coins)))
No problem. I have seen our coins all my life and didn't know squat about them... until I got hooked on Lincoln cents.
Besides, we all suffer from lack of knowledge about all kinds of things. I still don't know nearly as much as I should about cents but I keep learning everyday.
From what I understand it is probably a repunched mint mark (RPM). The mint mark is D for Denver, but it appears to be over another letter. The coin is very well circulated and has acquired some toning in its journeys, so it is very difficult to photograph.
My first photo turned out much too dark to really see the detail on the screen. This time I used more light and made it larger.
As far as the letter under the D, I am thinking is probably another D that is southeast of the top D. Whether it was tripled punched, I cannot determine. It is quite a thick D and could be tripled. It is all up for debate now.
Thanks for looking, and for the compliment about the photography. Good cameras make bad photographers have better results.
Corky