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!