Brophy's "Plans and Specifications of the LC Smith Shotgun" has a drawing on page 243 of what he lists as the Fulton Gun, which uses a Greener type cross bolt as its only means of locking the barrels, and it uses a sliding part in the barrel lump to lift the cocking lever(s). The Fulton did not use this locking method in production, but the gun in this thread might be an experimental model to demonstrate the concept. Hunter played with the concept enough to make at least one set of drawings.