Martin,
you have been researching 'GO for a number of years now, and I would assume you have exhausted most available sources.
Have you checked the Jane's for 1926, 27 usw?
What about the Fana de l'Aviation people?
Musee de l'Air Le Bourget?
The team that put Chronique de l'Aviation together (under Edouard Chemel) published in 1999 ? (a long shot, I know!)
brgds
Alan