The answer lies in SL 49-10 Dec 1977 “APU High Altitude Starting Improvement”. To summarise, Boeing added the vortex generator from April 1977 to improve inlet ram recovery and thereby improve the pressure difference across the APU during the flight. It goes on… “The vortex generator successfully reversed the former negative pressure difference across the APU so that aerodynamic cranking could assist the starter motor during in-flight starts.
The original VG did not work as well as expected so they added the trailing edge articulated flap which folds inboard 25 degrees when the APU is started and running. It is unfolded (and fared) when the APU is not running to reduce drag.
Boeing said that with these mods and a few others, the APU will start at 35,000ft after 3 hours cold soak.
S&L