a first cheek plate and a second cheek plate designed to be fastened one to a seat back and the other to a seat proper, said second cheek plate being mounted to pivot on the first cheek plate about a first pivot axis;

a locking device for preventing the first cheek plate from pivoting relative to the second cheek plate, said locking device comprising a first locking element coupled to the first cheek plate, and a second locking element mounted to move on the second cheek plate between an active position in which it co-operates with the first locking element to prevent the second cheek plate from pivoting relative to the first cheek plate about the first pivot axis, and an inactive position in which it allows the second cheek plate to pivot relative to the first cheek plate;

a non-reverse device for preventing the second locking element from moving from its active position towards its inactive position when torque is exerted between the first cheek plate and the second cheek plate, said non-reverse device comprising a cam mounted to pivot about a second pivot axis between an active position in which said cam presents a bearing surface coming tangentially into contact with a bearing surface of the second locking element when the second element is in the active position, and an inactive position in which said cam is spaced apart from the second locking element, so that it allows the second locking element to move from its active position to its inactive position;

wherein at least one of the bearing surfaces from among the bearing surface of the cam and the bearing surface of the second locking element is provided with teeth forming projections coming substantially into contact with the other bearing surface when the cam is in the active position.