10 and the rung 16 in the direction of the arrows a first the sidepiece 14 slides over the guide surface 40 and already catches against the detent 38 before the penetration depth of the rung 16 in the sleeve 10 has reached the distance of the walls 18 and 20 of the sidepiece 14 from one another fig4 illustrates a further exemplary embodiment of the inventive sleeve with the same reference numerals being used for comparable parts this embodiment differs from the embodiment illustrated in fig1 to 3 merely in that the detent 36 acts upon an engagement surface 38 that is provided on the outside of the inner wall 20 of the sidepiece 14 i e on that side of the inner wall 20 that faces the rung 16 the configuration of the resilient tongue 34 is effected by providing slits 44 that extend in the direction of the axis of the sleeve 10 at least two and preferably four slits 44 are distributed about the periphery of the sleeve 10 as a result of the press fit between the rung hole 12 and the sleeve 10 on the one hand as well as the sleeve 10 and the rung 16 on the other hand the rung 16.