Lever 1, turning about fixed axis A, has two symmetrical contoured portions a which alternately actuate plungers 3, sliding in guides of link 2. When lever 1 is turned, link 2 is shifted from one extreme position to the other with a stop in the middle position. Each extreme position corresponds to a position of lever 1 up against one of the stops b and a position of link 2 up against one of the stops c. Springs 4 apply forces to hold the device in its extreme positions.
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