Light Aircraft Association

It's not just bikes that use imperial units for brake hydraulics. All the classic VW ones I know of are imperial, though are listed with bores of 19.05mm, 20.64mm, 22.2mm, 23.8mm, and 25.4mm., which are 3/4", 13/16", 7/8" and 1", and I couldn't find any proof that precise whole number metric values exist.
I suspect hydraulic brake cylinders are standardised on imperial units, at least for vehicles, perhaps because the main players from decades ago set the standards, and the US market is so large.

I had a suzuki GT250 with one of those front brake reservoir brake lever and mirror mount, most motorcycles used that format in the mid to late 70s. broke a few my self.

Thanks for that confirmation, exraf. I'm not a motorbike man myself but it's perfectly obvious to me now from the mounting and the cut down lever that the mastercylinders could only have originated from a bike - lateral thinking is not my strongest suit! Certainly, the Honda repair kits I bought are exactly right.