Genesis angled valve for 3/4" necks. It's the one pictured here: http://www.scuba.com/blog/2009/01/ge...-valve-4495-2/

I'm not the biggest fan of these style valves and have replaced it with a different one, but I know some people like these for their o2 bottles so they can tell it apart from other gases by feel.

$25 shipped in the continental US.