That's the ring on the gooseneck- called a downhaul. Line goes to a cleat to hold the luff tight. In your very first picture it's upside down. That part pulls aout against spring tension and turns. An older way of reefing.Good afternoon Cecil, if I've understood the guys correctly the part we're missing is called a boom kicker I think. It's a round or flat piece of aluminum that has a knurled nut or thumb screw on it that slides into the mast slot below the gooseneck, and once tightened prevents the boom from completely sliding down when the main is lowered (found them on SBO store and west marine sites). The eyelet on mine should have a line that leads to a cleat lower on my mast to prevent the boom from riding up under sail (similar to a Cunningham I believe).