I have tried the following sites with no success:
https://www.aviafrance.com/recherche.php
http://www.antonakis.co.uk/registers/France.txt
https://immat.aviation-civile.gouv.f...ef_liste.html#
I think I saw somewhere that F-Wxxx is for prototypes, but with numerals following the W.
Not merely a prototype, but pseudonymous? The ultimate spy plane?