S&W Model 19:
1957: stamp model number
19-1 (1959): left hand thread on ejector rod
19-2 (1961): eliminate trigger guard screw
19-3 (1967): move rear sight leaf screw
19-4 (1977): move gas ring to cylinder
19-5 (1982): eliminate pinned barrel, eliminate recessed cylinder
19-6 (1988): new yoke retention system, floating hand
19-7 (1994): new rear sight, drill and tap, new extractor (some MIM parts later in change)
19-8 (1998): MIM parts including hammer and frame mounted firing pin.