66 - no dash - introduced 1970.
66-1 - 1977, changed the gas ring from the yoke to the cylinder.
66-2 - 1982, eliminated pinned and recessed, slightly lengthened cylinder.
66-3 - 1986, new yoke retention system / radius stud package / hammer nose bushing / floating hand
66-4 - 1994, change rear sight leaf, drill and tap frame, introduce Hogue grips, change extractor.
66-5 - 1998, change in frame design: eliminate cylinder stop stud/eliminate serrated tangs/change to MIM hammer with floating firing pin/change internal lockwork.
66-6 - 2002, introduce internal lock.
2005 - discontinued.
Source: S&W Model 66 - Gunpedia