the stops 33 or 34 and decoupled from the slide only during the period its movement when it is located in the end positions after a rotation angle of 180 degrees each time in principle the slide is arranged parallely to a side wall of the housing 20 and engages with the thin extension 21 parallely to the cover and shortly beneath this cover into the housing pot 21 in the area of the side wall at which the slide 30 is situated a second slide 40 is guided in the direction of a possible movement of the second slide 40 relative to the first slide 30 in the direction of the double arrow a the second slide 40 laterally comprises two rails 41 for guiding which rails 41 engage into grooves 42 of the first slide 30 which are open at one end in order to engage the second slide 40 into the first slide 30 as can be clearly seen from fig2 the one side wall 43 of the grooves 42 is higher than the other side wall correspondingly the rails 41 at the side turned towards the side wall 43 are higher than at the other side so that the slide 40 can be engaged only in one single relative position to the slide 30 into said slide the open end of the grooves 42 is situated at the front side 44 of the slide 30 the two rails 45 of the slide 30 in which the grooves 42 are.