Three parts only. Plus the button. Spring (ressort) under sight. Pin through receiver holds sight in place. Windage adjusted on the sight. Elevation by screw at very front of sight. The button merely keeps sight in the down position and is held in that position by an internal spring.
Put button and its spring in place. Push in to allow, next, sight and the spring that goes under it. Insert from rear. Then secure with the cross pin.