Well, the point was that the 2 two speed single stage Merlin was not commonly used in the Spitfire (like the Merlin XX in the Hurricane).
Basicly:
Single stage = one blower
Two stage = two blowers in series
Single speed = supercharger run single speed (or constant ratio)
Two speed = supercharger can run two different speed
=>
Merlin III, 45, 50 etc. = single stage, single speed
Merlin XX, 25, 28, V-1650-1 etc. = single stage, two speed
Merlin 60, 100, V-1650-3 etc. = two stage, two speed
In addition there were at least experimental Merlins with three speed system as well as variable speed system.
gripen