For dives deeper than 60-70 feet, or if I'm solo, I mount a 30cf pony to my steel 120. My main tank has an H-valve, so my secondary reg from that is what I would give to an out of air buddy.
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I expect you will have issues traveling with a pony bottle. You will need to empty the tank and have the valve loose for inspection. Although I understand you can take it on a plane empty, the tank may be consficated as the TSM employee may not know the rules. Call the dive op, see if you can rent a pony. Might be easier.
Tom