effort represented by arrow f 3 in fig5 tending to tilt the lock 16 about axes x 1 and x 2 in the direction of arrow f u2032 1 opposite to arrow f 1 the lock 16 is provided with a housing 167 for receiving the pin 183 with possibility of rotation while the web 184 is provided with a beak 186 intended to be engaged in a cavity 168 in the lock 16 oriented opposite the surface 161 in this way once the pin 183 is in place in the housing 167 it is possible to pivot the member 18 about the geometrical axis x 3 of the pin 183 to immobilize the member 18 on the lock 16 when the member 18 is in place on the lock 16 its plates 181 and 182 constitute two cheeks which isolate the volume v defined between the lock 16 and the arm 15 from the outside and in which the spring 17 and the pin 163 are disposed in other words the member 18 is an element for containment of the volume v which makes it possible to protect this volume against pollution and in particular against flock the member 18 also makes it possible to retain within the volume v a lubricant such as grease such a lubricant being able to be introduced in order to facilitate the articulation of the lock 16 on the arm 15 once the.