DG regs
Just had a quick look at the DG manual. P15 (provisions for dangerous goods carried by pax or crew) says that "non-flammable gas cylinder fitted into a life jacket containing carbon dioxide or other suitable gas in div 2.2, up to 2 small cylinders per pax, and up to 2 spare cartridges." are: 1) permitted in or as carry-on; 2) permitted in or as checked baggage; 3) permitted on one's person and 4) the approval of the operator is required (5 : notoc not req'd).
So, you are allowed to take it but need the airline's approval. If they lost it, obviously they will say that they had not given it. Good luck!