Thanks! I've had a little experience with that.
The aircraft serial no. is missing from under the left wing, as well. This could be due to the aircraft being painted as it was available between flights, or during a period of scheduled maintenance. The missing numbers could therefore be accounted for as temporarily normal, but the French roundels? This might be a BT-9 (or equivalent) built for the French AF that was towed over the U.S.-Canadian border after being built, then flown east for shipment. France might have fallen before the bird got home. BTW if you've seen "I Wanted Wings," this looks like the type of trainer initially flown in that film. It's sure got that North American vertical stab and rudder.
I've noted the yellow biplane trainers in the background of one shot. They are not the closed-cockpit Stearmans the Canadians used, I don't recognize the type.
Fun to watch the film and note where these installations were, they're not far from home near Detroit and we used to visit many of these places when I was a kid. Just didn't know so much happened there to influence events oceans away.