Aprilia gearbox output shaft 38-20001

[Raahauge]

I have recently broken the output shaft when gently putting the car away in the shed, it had quite a set on the splines so has suffered a shock at some earlier time so I judge that it was that rather than general fatigue which was the cause. I have replaced it with a spare but that is likely to be from an early car.
Does anyone have a drawing, or know if the shafts on the later cars were made out of better material? The car is now back on the road so I am not about to change it but I would like to understand it better.
I have mailed Cavalitto about price, availability and material but have had no response.