I’ve been rooting around in my “hope chest” (so called ‘cos ‘tis the last hope of finding wot I’m looking ‘fer) and I can confirm there are indeed fittings with a 7/16in. (11mm) x 19tpi thread, I know ‘cos I’ve found some. Fifty odd years wiv bikes and in the engineering game ‘an I’ve only just cottoned on!!! Dooh!
The BSP series came into existence for iron pipework but for finer types of piping it’s a bit on the “klunky” side. I imagine that the manufacturers chose to loosely adopt the BSP series, probably because they were already involved in pipe fittings and had some tooling handy, but modified the sizes to make a neater component that in fairness doesn’t need to be any bigger.
I’ve looked in several reference books but can find no mention of a standard for small pipework so these fittings should be regarded as peculiar to the manufacturer. There are certainly quite enough variations particularly in methods of sealing, olives, ferrules, etc. come in all shapes and sizes, certainly more than you could throw a stick at!
The down side is that taps and dies of this size are likely to be about as available as hen’s teeth so if all else fails it’ll be a screwcutting job and 19tpi is not the nicest pitch to deal with.
There is one other possibility, given that there is usually at most only five effective turns on fittings of this size and that there is only a 0,003in. pitch difference between 19tpi and 18tpi then the nearest and maybe more readily available thread is 7/16in. BSF, running a tap or die of this size down an existing thread will barely be noticed. If nothing else when a replacement parts has to be made because originals cannot be found then the making thereof will be that bit easier.
Anyway that’s the nearest you’re likely to get and what’s more they’ll never see it from the High Street!