With my micrometer I measure the external diameter of a male BSPP 3/4" to be 26.5mm.
However I have another male and female connector, it purports to be 3/4", it is parallel, and it measures 25.7mm external diameter. As a result it is not compatible with the 3/4" BSPP (it is smaller). The suppliers of this connector have called it "ISO 228, standard 3/4" but I am not sure exactly what it is. All I know is that it is parallel and that my 3/4" BSPP is too large to fit in (it seems by 0.8mm).
It is a 20" water filter housing, which has two ports, for input and output, both plastic, and there are two such housings interconnected with a male-male plastic connector, which is shown here
The only things I can think of is to take the piece you want to attach to a decent (independent) plumbers merchant and try everything they could suggest.
Failing that try a pool supplies merchant and see if they have anything which may fit - most pool fittings are plastic and can have a few size variations - the clue being the 'O' rings forming the seal rather than the thread
Yes it is a parallel fitting so the seal is mostly made with the O ring. However there is no male connector with an external diameter of 25.7mm. I keep looking.
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