Patented during the Civil War, the Mershon & Hollingsworth mechanism involved a clock-type spring wound with a fold-out handle.
The button on the winding plate is a lock-out latch that allowed "normal" revolver operation.
A manual hammer-block latch prevented the hammer from cocking while the spring was being wound.
A small tab on the latch engaged a step on the hammer. Note the hand-checkering on the latch assembly button.