The sleeve drive sprocket has been drilled by Bob to allow for a vernier adjustment. The 9 holes allow the sprocket to be fitted to the timing shaft in a number of different positions.
With 22 teeth not being visible by 3, three mounting holes would allow the sprocket to be fitted three ways, each time a tooth ends up in a different relative position. To refine this further, three sets of three mounting holes have been drilled.
Why? Well during final assembly the valve timing will need to be set. This will be achieved by setting the piston at the point where a valve opens, moving the sleeve into the correct position and then bolting the sprocket to the timing shaft.
Sounds so easy when you say it!