I think I have a 686-5 7-shot. No lock, but hammer and trigger are MIM. The trigger pull on this gun is actually better than a 686 6-shot I had previously without the MIM parts, but it does sound kind of "tinny" when dry firing. Doesn't bother me a bit though, the gun shoots great.
I think the competition guys running super light double action pulls require (or maybe just prefer) the frame mounted firing pin, primarily because they can use a longer firing pin to ensure reliable ignition (with appropriate primers, of course).