Webleymkv did a great job explaining the different models, so I'll tackle a few of the feature changes.
All of the models seem a bit overwhelming - today I learning the difference between a model 19 and a 19-3 with a pinned barrel and recessed chamber.
S&W eliminated the barrel pin from J, K, and N frame revolvers in 1982, and all centerfire Magnum revolvers had the cylinder counterbore eliminated at the same time. S&W collectors refer to pre-1982 Magnums as "Pinned & Recessed" or P&R. The K and N frame Magnums all got a new dash number when this occurred. For K frames, the first non-P&R .357Mag models were the M13-3, M19-5, M65-5, and M66-2. For all .357Mag N frames, the first non-P&R model was the -3.
Keep in mind that the pinned barrel and recessed cylinder don't make any measurable difference in how the gun shoots. It's mainly a collecting thing. P&R guns command higher values because they're older and more traditional, and many S&W collectors obsess about tradition.
The L frames were introduced in 1981 and had non-pinned barrels and non-recessed cylinders from the start. There is no such thing as a P&R L frame.
J frame Magnums were never P&R because the J frame was not available in .357Mag until the 1996 frame stretch.
FWIW pre-1982 K & N frames in
non-Magnum centerfire calibers (.38Spl, .44Spl, .45ACP, etc.) never had recessed cylinders. OTOH
all post-1935 rimfire Smiths, including today's production, have recessed cylinders to prevent case-head separations with soft rimfire brass.
Some other features:
MIM: In 1996, S&W started producing the hammer, trigger, and other lockwork parts of their revolvers using metal injection molding (MIM) rather than the traditional forging and case-hardening. They simultaneously redesigned their centerfire products to use floating, frame-mounted firing pins rather than hammer-mounted firing pins. (Rimfire Smiths have had floating frame-mounted firing pins since the early 20th century.) Collectors wailed, gnashed their teeth, and tore their garments at such blasphemy. Most of the rest of us got over it pretty quickly.
FWIW the K frame M13, M19, and M65 and the L frame M581, M586, and M681 got the axe shortly after the change to MIM. The majority of used examples on the market have the traditional case-hardened parts with the hammer-mounted firing pin. The M66 lingered for a few years longer and is more frequently found with MIM. Also, the 1996 J frame stretch corresponds with the introduction of MIM, so all .357Mag J frames have MIM parts.
Internal Locking System: In 2004, S&W introduced the internal key lock on all of their revolvers, and to avoid a major thread hijack, I'll leave it at that.
S&W simultaneously introduced the L frame M619 and M620 as de facto replacements for the K frame M66; these models are ILS-only.
