corresponding to only one of the said keys 35 35 39 they are closed at the top by the bush 38 fixed to the shaft 19 39 itself the fork 39 is inserted in sliding fashion inside the said circular groove 36 and is integral with the rod 40 of the actuating means consisting for example of the simple action electromagnet 40 39 so that when the latter is energized it moves the fork 39 in the direction of the arrow x with the keys 35 and 35 39 in the disengaged position see fig5 that is outside of the corresponding slots 37 and 37 39 when the said electromagnet 40 39 is de energized the ring 34 is pushed in the direction opposite to the arrow x by means of the spring 41 wound on the shaft 19 and compressed between the said ring 34 and the bush 42 fixed to the shaft 19 in this way the keys 35 and 35 39 are inserted into the corresponding slots 37 and 37 39 only in the corrected engaged position as a result of which the two shafts 19 and 19 39 become integral with each other again see fig4 in the only angular position permitted that is in the position in which the two groups of the stitching elements perform the same operating phase in synchronization the said keys are appropriately chamfered at their ends to facilitate insertion in the respective slots correct insertion of the keys in the appropriate slots is obtained automatically since when the drive shaft 19 rotates integrally with the ring 34 it is the spring 41 which pushes the keys inside.