Hill started a project to make Automag valves more efficient. I bought one of his original inserts for the X-valve, but never got around to installing it. His initial testing showed that he could get 1500+ shots off a 68/4500 tank when using it. He made a short run of the inserts and sold them. Unfortunately, not many other people have posted any efficiency increase data.
Since I won't get around to installing it, I'm selling it. Hopefully someone else will have the time and ability to install it and test it out (and get a lot more shots off a tank with it too). If you're interested, check out my sale thread here: X-valve efficiency insert