Mike, Is the button not a cap nut on a threaded rod from the lever? I could guess you would tighten the button to a snug hold on a hook and then move the free end of the lever out from the main shaft to lock the hook in place. That appears to be an early version of a rotary vise. Is there a shoulder on the shaft to prevent it from sliding right and left in the housing? I cannot figure what the purpose of what appears to be a machine screw to the left of the button is.