I think Dan-Air at one time used a variety of fixed call signs for non revenue flights -:
Danair 99 Papa Lime for positioning.
Danair 89 Papa Lima for engineering test flights
Danair 79 Papa Lima for training flights
I think there were others. (This example for G-ASPL)
Curiously I have just recently been listening to a recording (naughty I know) of London Radar for Sunday 4th June 1967 Transglobe Brittanias en route to Gatwick we’re using both reg and flight number on different flights.
Does anyone remember Berna Radio on Short Wave or H.F.? Listening to the Africa net?
Or Shanwick Oceanic on Short wave?
London Volmet on HF for early clues on possible diversions from LAP?
I believe ACE Freighters used ACE and last two of reg as call sign but what was their two letter code or SITA code ??
Be lucky
David