My guess would be a hole drilled from the grip through to the breech with that breech plug having a spring in it so you pulled it back to let a ball in when you turned the gun upside down, then when you let go of the plug it pushed the ball past the transfer port hole. If you had to unscrew it it would be a pain. I;m wondering though if it would unscrew as well for getting balls into the grip or there was another hole for that?