[camber]

Thank you very much Banks and everyone, I think this now gives the full picture! The document you have posted is the full explanation of how the system worked pre 100 octane conversion (rated boost=6.25psi, override boost is maximum possible).

From AP1590B posted by Lane and Banks:

261. Operation of the cut-off valve (14) substitutes suction for boost pressure in the aneroid chamber (2). This has the effect of rendering the control virtually inoperative, as the relay piston is held in the forward position, enabling the throttle to be fully opened by the hand lever at any altitude.

So-pre 100 octane drilled hole, when the red tab was pulled the boost regulator piston was held in the left stop so that no control occured and the pilot could pull maximum boost (e.g. approx 20psi at ground level), as stated in previous references.

The 0.093 hole was drilled through the boost cutout valve (presumably with the valve removed, otherwise you drill into the boost regulator face behind it!) I think the precise location of the hole is to avoid tapping into the supercharger inlet channel 12 (on Figure 45 in diagram posted above). Otherwise there would be strange effects on the boost operation when the cutoff valve was deactivated.

The drilled boost cutout hole (with the boost cutout activated) is basically a leak from the outside air to passages connected to the supercharger inlet. Although it is no longer a closed system, the drilled hole should be under suction and hence no fuel-air should be vented. In theory the motor is running leaner when on +12psi boost via the cutout (due to outside air leaking into the supercharger intake), this tiny backflow for control purposes compared to the total flow going through the engine would probably make no difference whatoever. The lack of fuel-air venting would be good news for any Spitfire pilots that had an incendiary bullet graze their cowling I believe!

Variable versus fixed datum

Another thing that I found really interesting was the variable datum control, because this seemed to allow the pilot to have a separate control for boost setpoint. However this system does not seem to have actually been used in any real Spitfires. Instead, the engineers attached the lever on the variable aneroid to the throttle system so in effect, the pilot's throttle lever was no longer a direct throttle plate control but an altitude independant boost selector that did not allow you to go above a maximum. Very elegant!

The initial single datum system must have been a bit odd for pilots, for instance at full throttle input on the ground (and the controller piston quietly disallowing actual full throttle to maintain 6.25psi boost), if you suddenly closed the pilot throttle input, nothing would happen initially! You would have to wait until the piston (really controlling the throttle) to move back.

The boost control piston/throttle level/throttle plate linkage

According to AP1590B, the linkage is not a linkage (as shown in my original post) but a differential gear instead! The mechanical logic seems to be the same though.

Cheers, camber