Found this on these forums:
The M686 started production in 1980. In 1986 the M686-1 had the radius stud package and floating hand. In 1987 S&W recalled both these models to change the hammer nose bushing and associated parts. If the modification has been done there should be a M stamped over Mod686 on the frame.
The M686-2 started in 1987 with the same hammer nose bushing change.
The M686-3 started in 1988 with a new yoke retention system.
And the M686-4 started in 1993 with a changed rear sight leaf, drill and tap frame (for scope mount,I think), change extractor and introduce Hogue grips.
The 686-4 is also available with the 7-shot cylinder. (The first I think).
The 686-5 marks the change to the flat hammer nose and new CNC frame, so the 686-4 is the last of the hammer mounted firing pin on the old familiar frame.
The M686 started production in 1980. In 1986 the M686-1 had the radius stud package and floating hand. In 1987 S&W recalled both these models to change the hammer nose bushing and associated parts. If the modification has been done there should be a M stamped over Mod686 on the frame.
The M686-2 started in 1987 with the same hammer nose bushing change.
The M686-3 started in 1988 with a new yoke retention system.
And the M686-4 started in 1993 with a changed rear sight leaf, drill and tap frame (for scope mount,I think), change extractor and introduce Hogue grips.
The 686-4 is also available with the 7-shot cylinder. (The first I think).
The 686-5 marks the change to the flat hammer nose and new CNC frame, so the 686-4 is the last of the hammer mounted firing pin on the old familiar frame.