The front sight on the first and second generation pistols (39/x39, 59/x59) were forged as part of the slide, so a 439 would require milling away the front sight and then milling a dovetail for a fiber optic front sight.
Well, a 5906 was offered with two different rear sight options... the protected “winged” adjustable rear sight and the fixed Novak lo-mount sight.
The guns with adjustable rear sight used a taller overall front sight BUT it would make more sense in production/manufacturing that the actual sight blade dovetailed in would be the same piece.
If you look at picture of two different 5906’s you can see how the front sight sits higher on the adjustable sight guns.