I have a dinosaur BOC reg (Regosaurus) that i had to adapt to suit a fire extinguisher.
The thread to the bottle was really thick and would not suit a normal CO2 set-up (as mentioned above by Mckenry).
I had it machined by a local trady for 40 bucks and works a dream.
Worked out a lot more affordable for me, but is obviously kinda ghetto.
Several years ago they changed the CO2 cylinder to the AS 'type 30' that we have now on the cylinders. Older regulators may still have the 'lpg gas' style fitting that was common amoung most industrial gases at the time (from memory nitrogen, argon and co2 all had the same cylinder fitting).
When the standard changed, BOC, etc made new inlet stems available (the bit that connects the reg to the cylinder) so industry could make use of their existing regulators. Search the forum for 'inlet stem' and you will get the BOC part number to convert the older BOC CO2 regs (or any with a 1/4" NPT high pressure inlet) to the 'type 30' now used on cylinders.
I've converted 2 regs over to the new inlet stem, they are around $35 (you can get a new chinese regulator for that, but it pays for itself when the poor quality results in it leaking and costs you a gas refill).