a user interface of a computer a computing unit takes from a memory the coupling position on the plate 41 that is assigned to the type and activates the leds 61 in such a way that the illuminating led 62 at the coupling position lights up fig7 b shows an alternative exemplary embodiment in which the leds 61 are arranged at the edge of the plate 41 here the illuminating leds 62 indicate coordinates for the correct coupling position of the fastening foot to facilitate the positioning fine lines may be engraved as a pattern of lines 63 in the plate 41 fig7 c shows a third exemplary embodiment of the visual indication of the coupling position in which a projected position marking 64 here a crosshair of a line laser is projected onto the plate 41 in order to indicate the correct coupling position alternatively a corresponding image may be projected onto the plate 41 by a projector fig8 a shows a front view of three extension frames 45 which laterally and upwardly extend the frame 45 as it is explained here in the context of fig5 a and 5b as can be seen from fig8 a each extension frame 45 is for its part equipped with guides and adaptations for the insertion of a test slider the extension frames 45 may be temporarily attached to the frame 42 if the distance between the actuating slider and the test slider is greater than was envisaged by the original design of the frame 42 the extension frames 45 may however also be pre mounted and able to fold away to the rear as revealed by the side view in fig8 b and the plan view in fig8 c in principle the adaptations 81 91 shown in fig5.