necks as explained below a top annular joining plate 16 bridges the annular support element 9 displacing element 10 and fixed element 13 and is removably fixed thereon by suitable fixing means 17 17 u2032 such as screws even if other conventional fixing means may be suitable as well when screws are used the fixing means 17 u2032 connecting the annular joining plate 16 with the annular displacing element 10 are preferably made of a screw bolt assembly and pass through slots that are made in the annular displacing element 10 in order to allow its displacement by rotation around the star axis once the screws have been loosened the annular displacing element 10 and the annular fixed element 13 comprise the gripper assembly wherein the grippers 12 and the gripper mechanism are carried by the annular fixed element 13 while the cam means 11 for actuating the grippers 12 are sustained by the annular displacing element 10 the grippers 12 comprise each first 18 a and second 18 b jaws both the cam means 11 and the jaws 18 a 18 b are pivotally hinged to the annular displacing element 10 and to the annular fixed element 13 respectively by means of suitable hinge means 19 19 u2032 19 u2033 the grippers 12 are positioned on the top side of the annular fixed element 13 while the corresponding mechanism is set on the other side of this annular fixed element 13 the cam means 11 are also set at the lower side of the corresponding annular displacing element 10 in order to interfere with the grippers mechanism as will be described below the cam means 11 are of a.