The M meant that it was modified by either the factory or a S&W trained armorer. I was one of those, primarily for LE. It required replacement of the hammer nose bushing, and hammer nose. It involved fitting these parts, and chamfering the bushing to factory specs. I still have the parts, gauges, and such to do it. We were paid by S&W to do it. The guns would lock up solid with the right kind of ammo.
I never experienced the problem, and we all carried L Frames. This was supposed to solve the problem. Final step was to stamp the M on the gun. Hope that helps. RG