THE ANSWER!!!
The blooper is that the drawing and the caption DO NOT MATCH.
The caption says that this is a drawing of a single-stage, two-speed supercharger for a Merlin XX
In fact, the drawing is of a single-stage, one-speed supercharger for a Merlin Mk II or III
The Merlin 2-speed supercharger is much more complicated than the 1-speed. There are selector forks running through the mechanism and there are two pinions on the impeller shaft, one for high speed and one for low.