The history of the post-World War 2 S&W semi-autos began from its famous model 39 pistol, the first productional Double Action pistol made in USA. The development ot the model 39 began in 1949, and first production pistols saw the market in 1955.
Basically, all later S&W centerfire semiautos are further developments of model 39. Model 39 was recoil operated, locked breech firearm, utilising Browning High-power type locking principle and double action trigger with slide-mounted safety switch, also working as a decocker when switched to "safe" position. In 1970, S&W introduced model 59, a high-capacity (double stack magazine) modification of the model 39. The other difference between mod.39 and 59 was that mod.59 featured straight backstrap while mod.39 featured curved backstrap. Those two models formed so called First generation of S&W semi-autos. Both model 39 and model 59 are discontinued from production since 1980 or so.
Second generation of S&W semi-autos was introduced in 1980 and was separated from 1st generation by introduction of the three digit model numbers and different frame materials. All 2nd generation semi-autos were built on mod.39 and 59 basis, with double stack magazines and traditional double action triggers with safety/decockers. Many 2nd generation handguns featured high profile sights with ajustable rear sight. All 2nd generation S&W pistols, as well as 1st generation ones, featured three piece grips (metallic backstrap plus two sideplates, made fromm wood or plastic).
The key to understanding 2nd generation model numbers is following:
First digit of model number - frame material:
4xx - aluminium alloy frame
5xx - carbon steel frame
6xx - stainless steel frame
Second and third digits of model number - capacity and frame size:
x59 - 9mm high capacity (double stack magazines)
x39 - 9mm standart capacity (single stack magazines)
x69 - 9mm compact size high capacity (double stack magazines)
Second generation also featured some odd models, such as mod.745 and mod.945 single action, chambered in .45ACP. Most of second generation S&W pistols are now out of production.
Third generation of S&W semi-autos was introduced in 1990 by four digit numbered models. The third generation also introduced ambidextrous safety/decocker levers and double action only and decocker only models. Other innovations were new chamberings (.40 S&W, 10mm auto and .45ACP) and new Novak lo-carry three dot combat sights. The three-piece grips were replaced by polymer, one piece wrap-around grips.
The 10mm auto pisols, built on large .45ACP frames, were adopted by FBI and some police departments, but didn't caught the market and were discontinued from production in mid-1990s. The .40SW caliber line, on the other hand, was built on the 9mm frames, and still in production.
Later, S&W improved the third generation with introduction of TSW (Tactical Smith&Wesson) modifications, that featured acessory rail, mounted under the frame, ahead of the triggerguard.
The key to third generation S&W model numbering is as follows:
First two digits - caliber and frame type (for 9mm only).
39xx - 9mm single stack magazines
59xx - 9mm double stack magazines
69xx - 9mm compact, double stack magazines
40xx - .40 S&W
10xx - 10mm auto
45xx - .45 ACP
Third digit - trigger type and frame size
xx4x - DAO
xx5x - DAO, compact
xx8x - DAO
xx2x - DA w decocker
xx3x - DA w decocker
xx7x - DA w decocker, compact
xx0x - DA
xx1x - DA, compact
xx6x - DA, large frame