Form the S&W forum for what it's worth:
Engineering and Production Changes Before 1957, produced as The
357 Magnum (see previous section).
• 27 (1957): Stamping of model number continued as the Model 27.
• 27-1 (1960): Change extractor rod, right hand to left hand thread.
• 27-2 (1961): Cylinder stop changed, eliminate trigger guard screw.
• 1967: 6 1/ 2" barrel discontinued.
• 1968: Delete diamond grip.
• 1969: Change to “N” serial prefix.
• 1975: Target trigger, target hammer,
Patridge front sight on 6" and 8-3/ 8" barrels, introduced with Goncalo Alves target stocks and case.
• 1979: 3-1/ 2" and 5" barrel discontinued, 4" introduced with red ramp and
white outline rear sight.
• 1980: Target stocks standard.
• 27-3 (1982): Eliminate cylinder counterbore and pinned barrel (27-3 made 1982 - 1988 plus 14 guns in 1999). Small change in cylinder length to 1.57"
• 1986: Discontinue nickel finish.
• 27-4 (1988): New yoke retention system/ radius stud package/ floating hand.
• 27-5 (1990): Longer stop notch in cylinder (27-5 reported made 1989 - 1992, plus 110 guns in 1997).
• 1992: Discontinue 4" and
8-3/ 8" barrel.
• 27-6 (1993): Hogue grips, drill and tap frame, change
rear sight leaf, change extractor.
• 1994: Model 27 discontinued.
• 27-7 (2000): Performance Center version/ 8-shot, built on the new N frame, with floating
firing pin and extended frame lug– Special.
Todd.