Abstract: A pair of catches (74) is mounted on a sliding carriage (20) which is movable forwards and back along a guide (18) between a rear and a front dead centre (C,F), the catches being pivotable backwards and forwards between a working position and a position of rest. In order to move the sliding carriage (20) along the guide (18), a crank drive is provided. This has a crank (30), which can be driven in rotation, and a connecting rod (40), which is connected by a crank pivot joint (36) to the crank (30) and by a connecting-rod pivot point (42) to the sliding carriage (20). A control mechanism allows the catches (74) to assume their working position near the rear dead centre (C) of the sliding carriage (20) for its forward movement and to assume their position of rest near the front dead centre (F) of the sliding carriage (20) for its backward movement.