The S is for San Francisco and the M is the designer Hermon Mac Neil. I can’t tell you much else other than it was minted in 1917 or later as indicated by the type two reverse with three stars under the eagle.
I'd call it a Variety 2 (Pedestal Date 1917-1924) because there is a reasonable amount of design visible but no date. Not a recessed date which might show the date at least partially. Wouldn't grade (no date) but AG-03 details.