FYI: The 686-4 is also a pre-lock: The below is just copied and pasted from Wikipedia so I take no credit or responsibility for accuracy.
686 no dash, introduced model
686-1, 1986, radius stud package, floating hand
686-2, 1987, changed hammer nose, bushing and associated parts
686-3, 1988, new yoke retention system
686-4, 1993, change rear sight leaf, drill and tap frame, change extractor, hogue grips
686-5, 1997, change frame design to eliminate cylinder stop stud, eliminate serrated tangs, change to MIM hammer with floating firing pin, change to MIM trigger, change internal lockwork
686-6, 2001, internal lock