I've done a bit of research into this and they may be designated as Material of Trade (review 49 CFR 171.8 and 173.6) if the aggregate gross weight of the cylinders on the vehicle does not exceed 440 pounds. Each cylinder may not exceed 220 pounds, gross. The cylinders would need to marked with the hazardous contents. The vehicle driver would need to be informed of the hazardous contents. You would not need shipping papers or vehicle placards.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.