Thought that I told everyone about R, SE and SP gearboxes!

The MC21 are all the same for all models.

The MC28 are all the same for all models.

The MC21 and 28 have the same 1, 2 and 3, but 4, 5 and 6 are longer on the MC28 than the MC21. This is probably a trick by Honda to make the 5 hp down MC28 accelerate away from the lights as quick as the MC21, and still retain the top speed.

However, in reality I think that it's a cunning plan by Honda to make the racers buy the 4th 5th and 6th gears cogs from an MC21 to shorten their 28 box up.