surface 25 rib like guide pieces 22 are integrally formed onto the sides of the insertion lug 21 the guide pieces 22 have two guide surfaces 22 1 oriented at an angle with respect to each other so that the guide pieces 22 taper from their connecting region to the insertion lug 21 toward their free end the inclination angle is selected in accordance with the angle between the flanks 13 3 the guide surfaces 22 1 transition into one another via a connecting section 22 2 in the region of or near the free end of the insertion lug 21 a support lug 23 is formed which is embodied as a transition section and like the guide pieces 22 is embodied as rib like the support lug 23 transitions flush into the guide pieces 22 and thus has the same cross sectional geometry consequently it has guide surfaces 23 1 inclined in relation to each other and in turn connected to each other by a connecting section the insertion lug 21 has a support head 26 with a bearing section 26 3 formed integrally onto it in the transition region to the bearing section 26 3 the support head 26 has an annular shoulder.