Honda part number 22887-MBB-006.
Bingo - I ordered this and it does work, yes 2 O rings were needed, possibly some tweaking as well, mine is leaking at the big O ring it looks like as of now.
But I didn't have the cap, the rubber gasket, the plastic cap under the cap LOL, and I still don't have the clip that holds it sideways from spinning open.
Anyway a 97 RM250 rear master cyl reservoir has the cap, the rubber gasket/expander bladder (or whatever its called) the plastic cap etc. So my hydraulic master cyl is now back in business, I have a 2000 BMW F650 mirror clamp holding the clutch side in, will find a GS one for the brake side and I'll be able to fit mirrors on the clip on's.