Ian,
If PAW is configured with no 'Reg' in the 'Flight ID' box, it will just transmit the auto generated (or manually entered) HEX instead. I take it that is what has happened here and that you looked up the Reg on G-Info from the displayed HEX?
Having dealt with a few posts of this nature recently, I'm starting to get a bit concerned that some users are very lax about correctly programming transmitting equipment in their aircraft. (See Brinzlee's recent posts in the Ground Station thread). OK, we are using a 'licence free' frequency, but we still need to 'play by the rules' if we don't want to attract the potentially unwelcome attention of Offcom and the CAA. Maybe the 'Quick Start' sheet needs to stress a bit stronger the importance of correct equipment programming before use and not transmitting false information (though in this case of course the information may not be 'false' - just 'missing').
Regards
Peter