exemplary embodiments in accordance with fig1 through 3 the lateral surfaces 34 each terminate with an inwardly facing bar section 36 that locks to the associated bar section 26 of the retaining part 14 also disposed on the stop part 16 are a total of four bolts 38 that are oriented transverse to the u shape and each is retained on the stop part 16 by means of a bolt support 40 one of the pivot levers 20 is oriented on an upward incline at an angle of approx 45 u00b0 between a bolt support 40 and a lateral surface 34 of the stop part 16 and is pivotably fitted to the stop part 16 this design with bolt supports and associated short bolts that do not completely pass through the u shape may also be advantageously selected as an alternative for the bolts 28 on the retaining part 14 furthermore disposed on the stop part 16 is a cover surface 42 with which the stop part 16 is supported suspended on the retaining part 14 in the exemplary embodiment in accordance with fig4 through 6 the lateral surfaces 24 of the retaining part 14 are not s shaped but rather are embodied straight with bar sections 26 oriented inward in this exemplary embodiment the stop part 16 does not lock to the retaining part 14 but is nevertheless precisely retained and guided thereon by means of the pivot lever 20 and also the cover surface 42 the exemplary embodiment in accordance with fig7 through 9 is embodied with lateral surfaces 24 and 34 that are embodied completely straight that is without inwardly or outwardly.