into the basic module 2 and not pushed away therefrom under working conditions furthermore the forward contact surface 12 is segmented and comprises two surface segments 12 a and 12 b which though spatially separated from each other by the opening 17 in the basic module 2 both still lie in the plane 15 however the contact surface 13 is in the form of a continuous surface in the plane 16 the two contact surfaces 12 and 13 are interconnected via the stopping step 14 thus the stopping step 14 comprises the area of the contact region of the basic module 2 relative to the interchangeable holder 3 which is situated between the contact surface 12 or the surface segments 12 a and 12 b and the contact surface 13 the transition between the stopping step 14 and the rearward contact surface 13 is formed by the side edges 18 a and 18 b of the stopping step 14 and the transition relative to the forward contact surface 12 is formed by the side edges 19 a and 19 b fig6 the stopping step 14 is thus likewise segmented and comprises the two walls in the form of a steps or surface segments 14 a and 14 b also referred to below as wedge flanks or stepped segments each of the two surface segments 14 a and 14 b is likewise planar and lies independently in a plane the two planes of the surface segments 14 a and 14 b intersecting in.