Picked up one of the rather nice Victor SR310 CO2 regulators. As discovered by Suicidal Tendencies.
Not cheap at around £170 from Leengate Valves in the UK, but they are near-perfect for robots:
If you're in the US, you can get them from BOC. See the specs, with a picture of them there as they come, with input and output pressure dials. There is also the Victor catalogue page. The body is 62mm dia, 130mm long.
I removed both dials - the input (high pressure) is not much use as the pressure in the tank is pretty constant - just blocked it off with a 1/4 blanking plug. The output dial is connected to a T piece with an output connector of a size that I didn't recognise, so I ditched the T piece and dial and a standard 1/4 connector screws straight in (black elbow at far end in pic above).
Discovered that the US CO2 connector is very slightly smaller than UK version - you can screw a UK nut onto a US tank, but ya cant get the US nut on this reg onto a UK CO2 tank/extinguisher. Fortunately, unlike some regs, the inlet pipe unscrews quite easily, so I just replaced the nut with a UK CO2 nut (the large brass lump at the bottom of the pic above) - you can get these from welding supplies shops.
Also visible in the pic above is that I have machined off one of the fins on the body. This is just to enable a straight access path to the pin that keeps the extinguisher valve open. We only fit the pin immediately before a fight and remove it immediately after to make it safe, so it has to be easy to get at!