Vito , here is the info you asked about, and what they mean
19 (1957): Stamping of model number.
19-1 (1959): Change extractor rod, right to left-hand thread.
19-2 (1961): Cylinder stop changed, delete trigger guard screw.
(1963): Introduce 6" barrel.
(1963): 50 manufactured with 2.5" barrel, serial range K544672–K544721.
(1966): Introduce 2.5" barrel as standard.
19-3 (1967): Relocation of rear sight leaf screw.
(1968): Delete diamond-insert grip.
19-4 (1977): Change gas ring from yoke to cylinder. (Not really an "improvement". It did make it slightly easier to clean if there was heavy leading, but basically was a cost-cutting measure.)
19-5 (1982): Eliminate cylinder counter bore.
19-6 (????): ??
1986 S&W model 66-2 6" Barrel
66 (1970): Stamping of each model.
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): Introduced internal lock.
66-7 (?): Two piece barrel and internal lock,