When push button 1 is pressed downward, pin a, secured on link 2, slides along a curvilinear slot to position a'. This is the indexed lower position of the push button. Link 2 is connected by turning pair A to the shank of push button 1. When push button 1 is pressed a second time, pin a is transferred from the curvilinear slot to straight slot x-x and push button 1 is returned to its upper position by spring 3.
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