Link 1, having lever a, turns about fixed axis A. Link 1 has projection b which engages lug c of latch 3 when lever a is turned. Latch 3 slides in guides e-e of link 2. The base has an annular member with projections f. In any of the engaged positions, slot d of latch 3 slides over the corresponding projection f. When lever a is turned in either direction, latch 3 is disengaged from the base and link 2 is shifted by spring 4 to its next position. Spring 5 applies force to hold the device in an engaged position.
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