stressed leaf spring 23 the eccentric disc 22 is abruptly rotated through 45 u00b0 towards the left in which case at the same time the cam projection on the cam spring 27 latches into the following latching groove 26 which is displaced through 45 u00b0 relative to the central latching groove 26 in that operating condition the leaf spring 23 is relieved of stress as can be seen from fig5 that is to say it assumes a straight position the eccentric disc 22 is symmetrical and has a larger diameter 30 and a smaller diameter 29 wherein the dimensions of the larger diameter 30 at both sides of the point of rotation of the eccentric disc 22 and also the dimensions of the smaller diameter at both sides of the point of rotation of the eccentric disc 22 are of equal length while in fig3 and 4 that is to say in the normal operating condition the two sliders 20 are in opposite relationship to the region of the smaller diameter 29 of the eccentric disc 22 and are thus not actuated in the 45 u00b0 position shown in fig5 the left hand slider 20 comes into contact with the larger diameter 30 of the eccentric disc 22 whereby the left hand slider 20 is displaced towards the left and as a result the left hand.