Right, I used to have an armie.
Easiest thing to do is take the valve pin out.
Unscrew the bottle off the main reg. If you look at the top of the bottle you'll see a blanking plug around the valve pin. You need a very large flat head screwdriver (or a large hex on much older ones). That unscrews out of the top of the bottle bit. Under that you will find a reg seat, pin and long spring.
Put all the bits in a zip lock bag as you do NOT want to get the seat contaminated and you are good to go. Effectively what you are doing is completley opening the bottle.
Whenever I have been quized I just showed the bits which where taped to the bottle and they knew I understood to keep it open.