I haven't the slightest idea. I have seen one on one private plane, but it was owned by a CAP instructor.
There is certainly nothing illegal or proprietary about that, and they are obsolete (CAP is not buying steam gauge airplanes anymore, and is gradually retiring those it has), so I'd suspect a few get surplussed here and there.
One would think this would be something Sporty's could sell for $500....or you could have fabricated for a lot less.
EDIT: Looks like these guys sell them, for $400.
http://www.rtaerospace.com/prices.htm
The photo looks identical, even down to the "remove before starting engine" label. As if you could start the engine with the mixture at cutoff.... Well, I suppose you could make it run for 10 sec or so.