flange portion 38 form stops which delimit the possible rotation of the clamping element to an angle which is just sufficient in order to rotate the shank 46 out of the position aligned with the end face 24 a into a position in which the circumferential wall of the shank 46 adjoining the flat area 50 rests on the end face 24 a for the rotation of the clamping element 48 a cross slot 57 into which the actuating end of the shank of a phillips screwdriver can be inserted is provided in the upwardly directed free end face of the disc shaped head portion 52 of the clamping element 48 it can also be seen in fig8 and 10 that in the free end region of the shank 46 of the clamping element 48 in the region of the circumferential surface of the shank 46 opposite the flat area 50 a low radial 58 projection is provided with which are associated curved low recesses 60 in the boundary wall of the strip shaped projection 40 facing the shank 46 in the insertion position of the clamping element 48 into the bores 44 the projection 58 engages in the recess 60 and thereby secures the clamping element against withdrawal from the bore 44 two walls 62 which are spaced from one another project from the flange portion at right.