Well this is what is stamped on the side of the tank...

5/8-18UNF CN(HS) 664963 AA6061
3.4MM 0.56KG 0.47L PH250BAR 0.35Kg CO2
TARE 0.67KG [Pi Symbol]0685-40C 82 2007/03

I guess that means it can take up to 250 Bar and the rest is thread size, weight, volume and material.

The ex-fire-extinguisher is in the green at 6 bar but the dial went up to 12bar; though that is in the red.

Cant find the soda stream bottle atm, will have to go hunting.