John that was the Magazine Cut Off. The powers to be believed the troop would waste ammo so insisted on this useless contraption. Load magazine, swing the MCO closed and feed one round at a time until you could see the whites of their eyes, or your officer so ordered then open it up and cut loose. You will some times find No1 with a cut/slot just under the edge of the ejection port where the MCO was. Didn't take long for the MCO to be scraped.