The system has a body comprising two opposite faces (11, 12) that are respectively defined in two approximate parallel planes (P11, P12). A control rod (20) comprises a rod part (21), and a coupling unit (30) is arranged to couple the body with the rod part such that a support face of the body rests on a counter-face (22) of a support and the rod part. A control unit is arranged to uncouple the body and the rod part when an anchoring element (15) e.g. screw, is provided in a position (Po2).