Offhand, I don't have the answer and my SCSW went up in smoke so I can't look but why would a 686 which is, by definition, stainless, have an S on it to represent a given fact?
It may indicate it is stainless. Other revolvers were stamped V on the rear cylinder face for this reason, due to different processes for blue/nickel/stainless steels.
It may also be an assembler, fitter or inspector stamp.
I am interested in what it means. I have seen a lot of 686's and have not come across this stamping. I was thinking assembly or inspection like MDog stated above. Let's see what others have to say.