I'm currently rebuilding a 1976 BMW R90S gearbox for a customer. The original splines on the drive shaft (left side in picture) still has some life, but the customer wanted it replaced since his R90S is being restored. The drive shaft is still available from BMW, although with a hefty price tag. These 18mm drive shafts were used in airhead models from 1974 (/6 models) to 1980 (/7 models). I've replaced quite a few of these on restored bikes, it's time consuming, but I've learned a few tricks along the way.