5 friction/clutch clearance issue in a 904

AJ/FormS said
Also when mixing 904/999 parts, remember that the output-shafts are matched to the planetaries.I mixed them up twice before I got it right.It stripped the splines right out of the planetaries 1st time I got on it....zing.

I know it's a really old thread, but i'm looking for more specifics on this info if any one can provide it as to the differences.