Hey newbie I have few with tripple MM I was told that it was MD but of course wait for more opinions. Dave
I do not see anything for 1974-S in my 2000 edition of The Cherrypickers guide. It could have been updated with your date and mint mark ? not sure ?
Neat find! I'm still learning about doubled dies and prepunched mint marks so I will not comment specifically on your coin. This link may help you learn more about RPM's.58243.html Even if the coin is found to be machine doubling it is interesting enough (at least to me) to put in a flip.
This is something @Fred Weinberg could comment on. Not sure if it could be Die Deterioration or Machine Doubling that affects only the mintmark.
Will post better pics, I took these at night. In Other pics it looks as though there is another sideways S under all the others !
To the best of my knowledge the mint marks were put on Die at the mints by hand in this era and sometimes had to be punched a few times to get the rite image and looks clearly to be just that ! IMHO But I'm no expert that's why it's here !
Mechanical doubling - note how the other 'S''s are flatter than the main "S" mintmark. Not an RPM, imo.