The data would have to be in-flight.
There are "anecdotal" suggestions out there that there might be something different going on with the wings besides the "normal" bending moment on some low-wing single-engined nose-pulled types that are beyond a certain weight and power (like WWII fighters).
I specifically want to know if a WWII era, or thereabouts, flight test exists that shows that the wing deflection was actually flight-tested on a single-engine prop fighter type and matched to the "calculated" wing-bending moment for a given amount of in-flight horizontal Gs. This specifically in horizontal turns, not dive pull-outs or other vertical maneuvers.
The reason I am asking this is that some WWII pilots claim they achieved their fastest prolonged and continuous turn rates at much reduced power levels, way below maximum power, because they were sustaining much smaller, slower in speed but quicker in rate, horizontal circles over very, very long periods of consecutive maximum-rate sustained turning (up to 45 consecutive 360s non-stop at ground level).
So it seems strange, as reducing power below maximum here was clearly not a matter of killing excess speed.
I am pretty sure these pilots were mistaken in thinking this was fastest in sustained turn rate, but as I said, I am only interested in knowing about the existence of an actual WWII-era wing-bending measurement vs in-flight G horizontal turning test.