[MrForbes]

The output shaft is actually 1/2" (0.500") at the "outside" end, and 12mm (0.472") where it passes thru the bearings. So, you could not assemble the gearbox with your plan, unless you changed the whole bearing mounting design.

The obvious solution to the problem is to spline the shaft and plate, but then you get into interchangeability and manufacturing issues.