first jaw 1 in the position with the casing 16 assembled to the first jaw 1 a notch 263 in the first longitudinal edge 261 provides a passage for the first radially projecting end 17 of the spring 14 the arcuate exterior peripheral slot 18 is in the exterior wall of the casing body 260 to provide a passage for and to allow angular movement of the second radially projecting end 15 of the spring 14 the other casing body parts are identical to those of the first embodiment namely the ends 19 and 20 open to provide the passage for the connecting spindle 13 the transverse faces 119 and 120 the rims 23 and 24 in this second variant the casing body can be molded in one piece alternatively it can be obtained by assembling a main part and a hoop similar to the hoop 162 from fig5 to retain the casing 16 of this second variant on the first jaw 1 means are provided for connecting it mechanically to the spring 14 or to the connecting spindle 13 either by branches at the ends of the u shape casing body which extend towards each other and bear laterally on respective opposite sides of the spring 14 or by end flanges having a hole through which the connecting spindle 13 passes in the embodiment shown in the figures the.