Andy wrote:Not forgetting, Roy, that the MC21 F3 rear wheel is actually an F3 part number, not an RS wheel!

The F3 wheel is actually very similar to the NSR Magtek.

That's right. The F3 wheel has a disc with a three bolt mounting.
A spacer goes behind the cush drive, which bolts on like the NSR Magtek.

The wheels I've got have a four bolt disc mounting.
The drive side spacer goes on the sprocket itself, as the cush drive is part of the wheel casting.