The punch (2) has a body section (12) comprising an outer surface that corresponds to a curved surface of a cylinder, and a snap-in section (11) axially offset from the body section with respect to a longitudinal axis (19) of the punch. The snap-in section has a convexly curved outer surface projecting outward beyond the outer surface of the body section. A head end (13) axially terminates the punch with respect to the longitudinal axis. A concave intermediate section (18) is arranged between the outer surface of the snap-in section and the head end. Independent claims are also included for the following: (1) an abutment (2) a retention insert.
