How many key pieces on rotor drive shaft?

I have a Mountfield Mercury 16. Model 420 SP. Year 1997. I have dismantled the rotor for cleaning and maintenance, but not sure how many key pieces are required. There is a slot for a straight bar type, and on the opposite side of the drive shaft there is a space for two crescent shaped segments. I only have one of the latter type. Is this sufficient and what is the design requirement ?