IMO the 639 is becoming collectable,
Its basically a stainless steel version of the Model 539,
the lighter aluminum frame version is the 439.
The 2 rear site options were the larger winged unit adjustable for windage & yardage or the smaller drift adjustable unit adjustable for windage by moving it left or right in the dovetail.
Interestingly although we refer to the 439/639 as 2nd gens the design had changed several times since it was introduced in the 50's , the original series had no model marking a short tang, short safety lever and non relieved left grip panel , a wide extractor , horizontal hole in the hammer spur and fully enclosed barrel end bushing,
On the 39 "no dash" the tang and safety lever got lengthened and the grip got a relief cut ,
To make things a bit confusing IIRC the 39-1 is actually sorta the Model 52 .
The 39-2 appeared around 1970 with a smaller extractor claw otherwise looking more like a 39, later versions exhibit other small changes such as the horizontal hole in the hammer disappearing, a change in the magazine release button and the enclosed barrel bushing becoming a horseshoe shape with associated frame machining differences.
The 439,539,639 have a new safety system that disconnects the firing pin instead of just arresting it and is therefore considered the 2nd gen, they also were the first to have an optional ambi safety lever and later versions have the serrated squared trigger guard.
The 3rd gens are not my forte but are easily recognized by their one piece plastic wraparound grip and 4 digit model numbers .
