Interesting 439, dont think I have seen another with that combination of features,
Have no idea of production numbers but the ones I usually encounter have the bigger adjustable rear site without the ambi safety lever , early ambi safeties had a phillips head retaining screw where later ones have the hex,
I purchased a brand new Model 459 around 1985 , it had the square trigger guard but no ambi safety,
never actually cared for the square trigger guard as it didnt fit in any of my older Model 59/39 holsters.
Back then I liked the bigger rear site but now I like the looks of the dovetailed fixed site.

Never cared for the Delrin grips on the 3rd gens but they got an improved dovetail front and rear site with Novak 3 dot sites which was a huge improvement over the fixed front site and factory dovetailed rear site on the 2nd gens, The first gens had a lower profile rear site that was adjustable for windage only although the rear blade could be swappedin the same manner as S&W revolver sites with different heights in black or white outline.