Abstract: An adjustment device including a rotary knob for driving a positioning stud cooperating with a ramp of an oeprating component, and a ring equipped with means for selecting the angular position of an end-of-travel stop, designed to limit the rotational movement of the knob to determine a maximum tripping threshold. The ring is covered by a cover plate having an opening for access to the head of the fine tripping threshold adjustment knob. The diameter of the second opening is smaller than the external diameter of the ring to inhibit any modification of the position of the end-of-travel stop after the cover plate has been fitted.