All US coins in 1965, 1966 and 1967 have no mint marks. They were produced at all 3 mints. If your coin truly has the letter S and it's not just something that looks like an S, it was put on after it was minted, either intentionally or accidentally. There's no premium for this. It's a quarter. The mint marks from 1932-1964 were on the reverse at the bottom, 6 o'clock. Since then, mint marks are on the obverse to the right of the hair tie. On your coin someone probably stamped an S there for some reason.
I close up photo would help. A possibility = dropped letter. Cannot be certain with this photo and the size/shape is not quite right. http://www.error-ref.com/struck-through_dropped_filling/
It is just the wrong shape for a dropped letter (I assume it is incuse?). I don't know what other alternative, besides a counterstamp?
There is a Peace dollar with almost the exact same "S" on another room Based on the photos of both, only, it's probably a counter-stamp of some type.