The tubular end part (2) comprises an elongated slit and is pref. somewhat longer than the dia. of the holding gripper (5). The slit in the tubular end part may extend vertically wrt. the axis of the holding gripper and widens towards the edge. Near the tubular end part typically extends a transverse pin (4) orthogonally to a plane between the end part and the holding gripper. In the latter may be located several apertures in different spacings from the tubular end part. The holding gripper may be also tubular.
