Cheated......opened them up and checked where the wires go (as I cant be sure whats going on where in each PGM).

Basically they are the same inside, same components, but with the red/yellow wire in MC28 motor replaced with red/white in MC21, and red white in 28 replaced with black white in 21

Although I think I got confused with the colours positions earlier, anyway, the long shot is I am happy now to fit it as is....but I am not repsonsible for anyone else swapping motors between bikes