back into its initial position the teeth of the latching pin 42 engaging the toothed structure 36 in a different place thereby once again locking the switch 22 in order to ensure a defined vertical position of the adjustment element 40 a stop 45 made up of two lateral projections is provided on the latching pin 42 above the two latching teeth and projecting laterally beyond them said stop 45 being in contact with the top of the bottom part 26 in the locked position the assembly of the switching unit 18 is simple and quick first of all the switching unit 18 is put together for this purpose in a first step the switch 22 is put in place at one end of the bottom part 26 as can be seen in fig3 the projections 30 and the tabs 32 are adapted to each other in such a way that the tabs 32 can first be guided past the projections transversely to the adjustment direction and can subsequently be slid between the bottom part 26 and the projections 30 after the switch 22 has been arranged in its linear guide the bearing arm 38 is attached to the switch 22 for example by means of a screw then the switch 22 has to be.