The 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 impress 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 is still in production. Later, S&W improved the third generation with introduction of TSW (Tactical Smith&Wesson) modifications, that featured an accessory 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
Fourth digit - Frame material (most models had stainless steel slides)
xxx3 - Aluminium alloy frame
xxx6 - Stainless Steel frame
xxx4 - blued or blackened frame & slide
[edited to fix typos]