5 in a conventional manner by two links 3 u2032 4 the base plate 1 consists of an elongated middle part 6 u shaped in cross section from whose bridge part an equally elongated ramp shaped elevation 7 has bent out which is equally u shaped in cross section the width of the ramp shaped elevation 7 is smaller than the width of the elongated middle part 6 so that to both sides of the ramp shaped elevation 7 on the bridge part of the middle part 6 shoulders 10 are formed which serve to support the lower edges of the legs 8 of the u shaped spacer 2 here the distance of the legs 8 of the spacer 2 corresponds to the width of the ramp shaped elevation 7 so that the spacer 2 is held on the base plate 1 free of any twist the lower edges of the legs of the spacer 2 are provided with tooth like protrusions 9 which engage the corrugations provided on the shoulders 10 of the base plate 1 the base plate 1 is provided with wing like extensions 11 symmetrically to the transverse centre line which form fixing flanges and which are provided with oblong holes whose centre lines are aligned with the transverse centre line of the base plate fixing screws 12.