two diametrically opposite notches 29b therein notches 29b are adapted to cooperate with the tip of a bolt 31 of which the base is provided with a lateral pivot pin 31a freely introduced in a performation made in the connecting rod 25 in the immediate vicinity of the edge thereof a spring 32 abutting against a stop 33 mounted against the connecting rod 25 tends to urge the pivoting bolt 31 so that is tip engages in one or the other of the two notches 29b resiliently ensuring the angular immobilization of the connecting rod 25 with respect to the eccentric 29 and vice versa each actuation element 34 a 34 further comprises a driver plate 34 of which the periphery has two diametrically opposite notches 34a therein with this driver plate 34 there is associated a pivoting hook 35 mounted on a lateral pin 29c extending from the plate 29a of the eccentric 29 a spring 36 bearing on a stop of the plate 29a tends to engaged the norse or tip 35a of the hook 25 inside one or the other of the two notches 34a of the driver plate 34 which is thus positioned angularly with respect to the eccentric 29 at that same time as it is.