Some ships had gas bottles attached to external (and internal) bulkheads. The CLGs had nitrogen bottles everywhere - dry nitrogen was used to charge hydraulic accumulators. We also had , oxygen and helium bottles. These bottles were all the same size. For a while they were color coded with the entire bottle painted a color specific for the gas. Then they were all painted green with a small color band at the top below the removable cap.
The ships also had acetylene bottles attached to external bulkheads. These were slightly shorter than the other bottles but larger diameter. Ours were painted yellow but I have also seen black acetylene bottles.
You can see examples of how they were stowed in pictures on this page:
http://www.okieboat.com/CAD%20superstructure.htmlPhil