New MBT vent valve pieces.
Here are all of the parts (excluding o-rings). The thick stainless piece will be welded into a hole in the conning tower. Next, the valve stem slips into the stainless piece from the outside. After that, the valve screws onto the stainless piece, with the valve stem interfacing with the ball inside of the ball valve. Finally, the handle goes onto the valve stem from inside the sub.