the lower end of the frame 25 glides either initially without seating of the frame 22 on the upper edge 19 or with seating on the upper edge 19 on the surface 8 in the upward direction until it comes to rest at stop 11 in an embodiment not shown whose dimensioning is chosen to be somewhat different than the embodiment represented in the drawing for example when the stop 11 and thereby the plate section 5 are provided in a more outward direction in this position the inner surface of the frame 22 seats on the upper horizontal front surface 19 of the plate section 5 or 10 and the form component 21 hangs approximately perpendicular in a downward direction in the event that the form component is then further pivoted inwardly from this position the pivot motion takes place about a pivot axis extending along the front surface 19 this pivot motion is limited in that the frame of the form component seats on the edge of the base plate 3 or on the support 1 during this continued pivoting the foot 25 is somewhat lifted and is displaced upwardly to the stop 11 the dimensions can be choosen in such a fashion that the foot 25 distinctly engages the stop 11 at the end of the pivoting motion so that the form component 21 must be somewhat raised to free the foot from the stop 11 embodiments.