the hitching ring 3 the articulation of the threaded rod 17 on the base 4 and the bearing of the sleeve 12 on said base 4 as a variant the nut 3 b may be replaced by a tapped hole in the upper face 12 a of the sleeve 12 in the exemplary embodiment depicted in fig8 and 9 the base 4 comprises two arms 40 a 40 b that are superimposed in their central part drilled holes 40 c provided at each end of the arms 40 a 40 b are intended to have self drilling screws passing through them for example the arms 40 a 40 b are able to pivot relative to one another for adjustment purposes to this end oblong slots 41 42 are made in each arm 40 a 40 b approximately in the central position the oblong slots 41 42 in the arm 40 a thus at least partially overlap the oblong slots in the other arm 40 b the extent of such an overlap depends on the relative pivoting of the two arms 40 a 40 b the clamping means comprising for example the yoke 15 pass through the superimposed oblong slots 41 42 in the two arms 40 a 40 b and thus allow the two arms 40 a 40 b forming the base.