The insert corner has a bisector plane (O-O) perpendicular to the base of the insert.

The light entrance is arranged in the bisecting plane of the first and second directions.

The insert corner (6) and cutting edges (5) are mirror symmetrical about the bisector plane (O-O).

The pivot portion and the working portion of each jaw may be on opposite sides of a bisecting plane.

Two-way control of actuators is achiev ed by using a bisected piston system.

Each branch runner (202) receives material having similar conditions from side-to-side of a bisecting plane of that runner.

The faces (4, 6 and 8) are also connected along the bisector plane Po of the angles (2) but form together an angle B of 120 degrees.

The small faces (3, 5 and 7) are connected on the bisector plane Po of the angles (2) of the sub-assembly and form together an angle A of 90 degrees.

The irregularities of the upper and lower faces of the block are preferably symmetrical about the plane bisecting the block intermediate the outer and inner faces (13, 14) of the block.

The rail support device (51) is symmetric with respect to a crossing angle bisector plane (79) which is a plane including a crossing angle bisector line (81) and developing in the vertical direction.

Preferably, the first and second reflecting sides of the reflector (25) are substantially symmetrically disposed about the longitudinal bisecting plane of the reflector (25) when the reflector (25) is in the deployed position.