"I think I saw somewhere that F-Wxxx is for prototypes, but with numerals following the W."
Flightless Parrot: F-Wxxx is used for flight testing, after which the registration reverts to a "normal" sequence, but still keeping the last three. So we need to see if there is a regular registration (eg: F-BLDE or others, but that was a Jodel). I've looked but find nothing.
What a challenge!
Laurence