I've got two wheels, an HRC wheel from (I think) an RS250, which fits, and a bog standard-looking front, which doesn't fit. The spacers could be from the RS250 which would explain everything, but I don't want to order some NSR parts from Honda to find out they're the same as what I've got when they arrive.
The standard-looking wheel is narrower (bearing to bearing).

Aha! So what I need to measure is the spacers. Anybody got any spacers lying around they would be happy to measure for me??

I've got another bike which I can take the wheel off, but that's got RS250 forks. I suppose that I can just measure them and see if they're the same. *sigh* I think I might be able to work this out somehow. Wouldn't have got there without being prompted by your suggestion though! SO thanks MAtt.
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MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-