fire extinguisher leaking

[Matheusilla]

Threads per inch does not rely upon the nominal size of the piece that they're on. NPT encompasses many different thread pitches.
NPT (national pipe thread) really only describes the threads as being on a taper. There are also straight threads on piping as well; they are not NPT.

For any given size of part there may be as many as 4 or more different thread sizes.

The threads on most of the fire extinguishers I've seen are a whole lot bigger than 22 threads per inch, they are more like 10 TPI.

If it's really that important, whoever wants to know should buy or borrow a pitch gauge (they're less than 5 bucks). Get the imperial and the metric set because you never know what you have until you measure it properly.