Turns out that Lindbergh's method was nothing particularly esoteric I've found. Apparently normal squadron practice was to fly at 2,200 to 2,400 RPM with mixture auto rich. Lindbergh introduced the practise of 1,600 RPM and auto lean.
2,300 RPM was the change over point for auto lean/auto rich operation according to the manual. Lean allowed a max of 34.5", rich 38" at 2,300. The manuals provide data for auto lean cruise, so wonder why the squadron didn't use it?