First off-Welcome to the forum.
The M stamp has been discussed here over and over.
Anyhow, the 581,681,586,686 no dash and dash 1 OCCASIONALLY exhibited primer flow back around the firing pin due to a very slightly over sized hole in the firing pin bushing. Didn't happen often or on every gun, but it happened enough that S&W issued a permanent recall to modify the bushing and hammer nose/firing pin. All revolvers sent back to S&W for this recall modification received an M (Modified) stamp to signify it was done. No effect on value and now you know for sure your revolver will fire all types of ammo.
I have an un-modified 686 and have had ZERO problems. Also have an M stamp 686-1. Both are great revolvers. Enjoy yours.