I've learned to fill and interpret "N" only when the flight is being flown under any AOC, (and as long as the M-registered planes cannot fly under any AOC), the FPL should be filled as "G".
Company's Own-Private Business its not Non-Scheduled AirTransport neither "under" AOC operations. However thats an issue with the ATC and its FPL organizations/authorities, so may exist different interpretations as already expressed here.
Tanx for your opinions and infos.