I've never seen a vent like that, but it looks like a very good one. If you can't find the joker valve and need to replace it, I suggest you use one made by Scot pumps. It looks like yours has a 1/8" male pipe thread so the Scot 109-13-VB
-18 should work. http://www.scotpumpmarine.com/CN/CN-10913.pdf
Just re read your post, if the thread is 1/2" ignore the above.