Bart...
The original Model 39's were steel frames, designed for an upcoming US Army trial to replace the 1911. That was in the early 1950's. Calmer heads prevailed, citing the huge numbers of 1911's already in service and that they were completely adequate for the role of a handgun in military service. S&W then tinkered with the design for the civilian market and brought out an aluminium alloy framed gun around 1954.
These early alloy-framed "Pre-Model 39" guns had a shorter slide mounted safety/decocker, shorter tang, a unique frame back strap, unique left grip, no trigger play spring and on the right, a long, wide extractor that was prone to breakage in heavy use. These guns were not yet marked "Model 39".
Steel framed guns were actually produced first, but the frames were never assembled for civilian production until 1966.
The Model 39-2 was brought out about 1971, shortly after introduction of the "A" prefix letter in the serial number. The 39-2 featured a redesigned shorter, narrower (pivoting) extractor, a new guide rod hole in the barrel bushing and a redesigned feed ramp. The improvements made the 39-2 rock solid in reliability.
I own five variants, including a 3914NL compact. These are fabulous guns that are very under rated. The grip-to-hand fit is exceptionally nice and many women find them very comfortable to shoot.
Model 39-2 pistols made from 1972 to 1982.
Circa 1967, S&W developed a 14-round alloy-framed version for the US Navy that featured larger sights, protective wings on the rear sights, a threaded barrel to accept a suppressor and a slide-lock to allow the gun to be fired in single-shot mode with the slide locked to reduce noise. The approximately 200 guns made were called the Mark 22, Model 0 by the Navy but known by their intended use as a "Hush Puppy" as they were used to silence guard dogs in Vietnam. This model was use to develop the civilian Model 59 double stack pistol.
Hush Puppy Pistol - Mark 22 Model 0