[Graham Boak]

I believe that these habits were common but not necessarily uniform throughout.

I would not normally expect an aircraft to be operated by the other flight, but operational needs would rule. The letter would not be changed for what would be seen as a temporary loan. The supply of fighters in the UK was such that no unit's flights would remain unbalanced in numbers for long enough to bother. An individual fighter may have been transferred from one flight to the other after a servicing period, but I don't know how common that might have been.

As long as enough Whirlwinds existed to maintain the unit at full strength, then I would expect them to continue with whatever the squadron policy was on such matters beforehand. But what that was on the Whirlwind units I don't know. With such a small force, it would be normal to have aircraft go away from the unit for major servicing rework, and then return: a Spitfire or Hurricane would not expect to go back to the same unit. In which case it may well not end up with the same flight, but the old letter would probably have been removed anyway, certainly if the aircraft had been repainted. I think you would have to look at the individual aircraft histories, and as many photos as you can find.

Normally each aircraft (at least for Spitfires and Hurricanes) would have a fitter and a rigger. Armourers were a unit asset, as were instrument fitters, who would normally work indoors.