A base plate is formed having a pair of guide projections projecting in the thickness direction thereof and an auxiliary projection situated between the guide projections. An internal gear is provided on the inner surface of an offset portion that is formed on a movable arm. An external gear on a latch member is in mesh with the internal gear. The guide projections and the auxiliary projection are fitted in the offset portion. The latch member is interposed between the first and second guide projections so as to be slidable in the vertical direction. The latch member can be moved to be disengaged from the internal gear by means of a cam member that rotates integrally with an operating lever. The auxiliary projection is inserted in a hole in the latch member and arranged on a line connecting the end portions of the guide projections.