according to a construction method as illustrated in fig1 7 the supporting body 11 is made up of two exactly equal elements or symmetric half bodies or shells 15 they therefore can be made advantageously from the same mould using die casting sintering or a forging systems in any appropriate material the two half bodies 15 are placed one facing the other they fit one into the other and are fixed together each one has a support plane 16 from which an intermediate part rises 17 with two sides 17 u2032 which is topped by a straight flat section 18 the supporting plane 16 forms a support base 16 u2032 and on the two opposite sides it has two ears 19 protruding from the intermediate section 17 each one having a half hole 20 perpendicular to the support base 16 u2032 and open towards the opposite semi body at the height of each half hole 20 a semi hexagonal recess 20 u2032 can be formed a cylindrical cavity 21 has been machined at the height of the intermediate section 17 the axis of which is parallel to the support base 16 u2032 the cavity 21 is closed by a bottom wall 22 in which a hole 22 u2032 is provided and opens towards the half body opposite in the top part of cavity 21 there is a slide surface.