The two gripping arms are held in shafts with first and second actuating levers (5,5A) positioned side-by-side in a mounting plate. The axes of rotation of the shafts (4,4A) are parallel to each other. An actuator or locking lever (7) pivots on a shaft with a cam (6). The cam moves the first actuating lever whose end is shaped to bear against the second actuating lever.