I found an old tap(pictured below) and opened it up to reveal this threaded plug thing that makes the tap screw and let water in. If you have a tap similar to this take it all apart and un-screw all the fittings.....
You will be left with a single fitting that is around about 16-17mm and about 7-9mm. But on the inside of the 16mm part is 1/4"(6mm) threads, so cut the 7-9mm threads off and grind it down so its smooth,now you have Fire Extinguisher threads on the outside and 1/4" threads on the inside. Simply thread it into any FE(most should work) and then tap a 1/4" thread into the fitting and then tap a 1/4"- whateverthreadyourusing and thats it.
Tap with fittings removed(Left fitting is modded, right fitting is non-modded)

Left Fitting has top cut of and grinded down revealing 1/4" threads, Right fitting is straight from the tap no mods made.

Tell me what you think and if it works out for you