You sure about that???
The supercharger switch is a three position spring loaded switch (Low,HighAuto)
You can manually switch to High at anytime as long as you hold the switch in that position. :book:
Also from the P-51 flight manual (Page 30)
"The supercharger works in either low or high blower mode, selection of which can be automatic or manually set by the pilot. In normal operations, high blower mode starts automatically from 14,500 to 19,500 feet, depending on the amount of ram air being delivered through the carburetor. The supercharger increases the blower-to-engine compression ratio from a low of 5.8 to 1 to a high of 7.35 to 1.
The supercharger can be controlled manually by a switch on the instrument panel. The switch has three positions – AUTOMATIC, LOW, and HIGH."