The operation member (11) has a central opening (17) with a periphery (18) and at least one protruding element (20) arranged on the periphery and extending into the opening, which is surrounded by a protruding rim (21). The protruding rim is interrupted along the periphery by at least one gap (22). If more than one gap is provided, the gaps are arranged such that the pattern of the gaps does not comprise a symmetry that would allow a rotation different from an integer multiple of 360° around a central axis of the opening to map the complete pattern of gaps onto itself. The operation member may be combined with a drive member comprising abutments in positions corresponding to the positions of the gaps.