Genghis,
I used to have a full rebuilt Merlin in my reception office when I owned a factory with assorted offices, the Merlin I had was a two stage two speed Supercharger type, just below the main air tube going into the intake manifold (looking from the rear) was a quadrant of levers that would have been connected by rods and bell cranks back to the cockpit controls, but it seemed that the second stage S/Charger would be activated by the same pushrod that activated the first stage (or speed), the same quadrant of levers were all connected as well to two cranked arms to either side of the mopping bucket sized Carburettor. So it always seemed to me that just one lever would operate the functions of stepping up and giving extra motion lotion, and more HP.
All these levers and bell cranks also were connected to a water injection system as well as the a small fist sized fitting at the front of the engine that I imagine was also connected with the propeller speed.
Who ever perfected that system possibly went mad.
I had many people come to see the Merlin, or possibly my well endowed Secretary but no one ever did satisfy my question of how it was all controlled, most were old Flyers who came to touch and remember things in past years.with a few philistines who wanted to know if it would fit into a car!!.
My regards
Peter R-B
Lancashire