I finally found my thread pitch gauge (hey its not easy when you are home and away as it were!)

They do have both the 20 and 16 pitch (and while that's a standard these are barrel threads and it could be non)

I could not remember if those were or not. Once in a blue moon do I need to check that sort of thing (machine shop would all the time)

So pull the barrel or get the gauge, then decide which way to go.