I purchased my yacht in Qld, Mollolabah, from a dealer, fitted with a rusty gas bottle and a fresh gas certificate.
The bottle is - one of those 'swap@go' jobs - usually not very rusty but as it sits in a dedicated gas locker with below water vent tube, is in an always moist environment.
This days l plug the tube lightly with a vine cork to keep moisture out and the bottle dry. (it is on the transom, an independent, dedicated gas locker with it's own large lid, before you chastise me for the blocking cork)
The locker is keeping the bottle separated from the bilge so there is no danger of being gassed if something goes awry.
The idea of a S/S or rust resistant bottle is logical, keeping it certified would be a headache.
To buy a S/S bottle according to the web site quoted above - my dear Jode - is indeed very expensive! Just a tiny 1.3 liter bottle $365.- ! is anything but cheap! My size, 4.5 liter would be $460.- isn't cheap either. In ten years time one must have it checked.
Could go for a cheaper solution like galvanised bottle for the quarter of the price $120.- a pop according to Gameco.
To 're-certify same $40.- including refill.
Btw, nobody mentioned that a 'swap@go bottle would be illegal!? Neither in Qld nor in Nsw.
Aren't they galvanised, anyway?