In order to give non linear displacement of an adjusting member 4 the connection between the actuating element (14) and the adjusting member (4) comprises a driving lever (1) connected to the actuating lever and pivotable about a first fixed pivot (7) and a driven lever (3) connected to the adjusting member and pivotable about a second fixed pivot (9) parallel to and spaced from the first fixed pivot. The free end of the driven lever has a pin (2) which is received in a slot (1c) in an arm (1a) of the driving lever. IMAGE
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