to permanently bear against the free edge of the disc 9 so as to engage every time the shaft 2 stops inside one or the other of the two notches 9a the hook 10 is controlled against the spring 12 by the end of two selector levers 13 fig1 pivotally carried by two fixed pins 14 oriented in parallel to shaft 2 these pivoting levers 13 are coupled to each other by a connecting rod 15 and one of them the righthand one in fig1 comprises an extension on which are articulated two rods 16 which are displaced transversely to their axis by two needles 17 controlled axially by a programmed reading device 18 operated electronically or mechanically the transverse displacement of the rods 16 under the effect of the needles 17 enables the rods in conventional manner as a function of the program incorporated in the device 18 either to cooperate with two push elements 19 with reciprocating movement consequently ensuring actuation of the two pivoting levers 13 or to escape the action of the push elements 19 in that case remaining immobile it will be observed that a spring 20 interposed between one of the levers 13 and the frame 1 tends to urge the end of the lever 13 with which it is associated in the direction of the shaft 2 and on the contrary to maintain the end of the opposite lever 13 spaced from said shaft so that the action of this spring 20 is exerted on the rod 15 in a direction opposite to the force exerted by push elements 19 a fixed stop 21 limits the pivoting of.