A difference between the prior art and the present invention is that the applicant has added two stop screws. The two stop screws are positioned in a track adjuster and extend through the retainer's riser side so each stop screw contacts a corresponding terminal end of the brackets when a lever is (a) in the relaxed position or (b) moved up in relation to a musical instrument. That way stop screws can be adjusted to maintain a predetermined minimum distance between (a) a moving plate's second aperture or second perch and (b) the track adjuster's perches.