Hi Tug,
If it helps, the Camel File confirms that, initially in 3Sqdn, the flight letter was typically repeated in white on port upper mainplane and "on some machines", was also carried in black under the lower starboard wing. So I'd suggest you are fairly safe with the upper wing and unlikely to be challenged if you opt for the lower wing in black.
If you want to let me know the serial I can check if there is anything on the particular machine.
Cheers
Sandy