The original 927 M39's were steel framed guns intended for military testing and later civilian/police use. These guns were actually not even completely assembled until 1966. These stem from discussion the military had about a 9mm service pistol. S&W produced the steel M39's but the military project never materialized so they were shelved.
Alloy framed pre-39's were made in the 1950's, after the steel frames, but were actually sold on the civilian market. These early 39's (starting at s/n 1001) were slightly different than the 39-2. Differences include:
- Smaller safety lever
- Smaller tang behind the hammer
- long extractor
- a unique frame backstrap
- no trigger play spring
- unique left grip panel
- No model number stamping on the frame
- the infamous "humped" feed ramp on some models
The Model 39-2 appeared approximately 1968-1970. The -2 model uses a shorter extractor because the previous type were prone to breakage. The feed ramp "hump" was eliminated, the safety lever and rear tang enlarged and other minor changes.
The humped feed ramp was blamed as a source of feed reliability problems in some guns. Others have thousands of rounds thru them without an issue. There's no doubt, however, that the 39-2 with it's straight beveled ramp is a more reliable gun.
It is also obvious that post-war S&W engineers used the Walther P-38/P1 mechanism as their starting point. There are at least 10 points of similarity in the mechanisms, including the slide-mounted safety design. The Walther design proved robust and reliable in WW-II conditions so it was a good choice to use as a model.