It's a "no dash", meaning that it's the 1st iteration of the model. Subsequent model changes would be indicated by a dash and a number following the 686, as in 686-1. The M indicates that it was returned to the factory, per a recall, to install a new firing pin bushing. SOME (not all) 686, and 686-1 models experienced primer flow problems with .357 ammo, that would lock the gun up and render it useless. I can't help you with the date of manufacture, but I think that the S may simply mean that the weapon is made of stainless steel. Someone will be along shortly to help you with the DOB......