them with carriers 2 installed at the head 28 the named support plane 32 absorbs the vertical forces to be transmitted from the carriers 2 onto a vertical support since the support surface 6 of a carrier 2 is ultimately supported on the support plane 32 of the head 28 which will be explained in the following spaced apart perpendicularly from the u shaped section 30 the support plane 32 has a respective coupling lug 34 which is curved downwardly in u shape at its two end regions remote from one another so that a receiving plane for a head plate not shown of a vertical support is defined between the coupling lugs 34 facing one another which extends parallel to the support plane 32 a head plate of this type can be pushed in the longitudinal direction of the u shaped section 30 into the coupling lugs 34 until it abuts two abutment members 36 which are made integrally with the u shaped section and extend downwardly starting from this into the named receiving plane for the head plate of the vertical support only one of these abutment members 36 can be seen in fig1 a spring member is fastened in the hollow space surrounded by the u shaped section 30 the end sections 40 of the spring member 38 are curved in the region of an opening 42 of the u shaped section 30 and in the region of an end face end of the u shaped section 30 such that the spring member.