You fit the filler nozzle to a camping gaz container which has a rather unusual thread I can't remember (the older style short fat ones not the 8 inch long ones that are common now). Look around 16mm model railway websites and forums and the modelboatmayhem forum for more details.
The nozzle mates with the horribly expensive filler valve. To fill you hold the canister upsidedown so itis liquid gas that goes out and have to vent the receiving tank to let it in so not while still alight. Try to size the tank so it runs out before the water as a gas flame is hot enough to distort an empty boiler if not actually melt silver solder.
Copied the following out of the Pollyengineering catalogue page 39
Filler nozzles, valves, jets, etc
Nozzles for refillable gas container (fits screw top canisters); gas canister valve, gas filler valves (Ronson
type), gas jets,
Filler Nozzle for refillable gas
container fits screw top canisters £12.90
Gas Canister Valve £19.60
Gas filler valves, Ronson type £5.10
Gas jets Sizes 5, 10, 15 £3.60