Link 1, having lever a, turns about fixed axis A. Link 1 has projection b which engages shoulder c of latch 3 when lever a is turned. Latch 3 has pin d which slides in a radial slot of ring 5. The base has slots e. When lever a is turned, projection b of link 1 engages shoulder c of latch 3, disengaging the latch from the base. At this link 2, together with ring 5, is shifted by spring 4 to its next position.
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