There is a separate ba reference which appears on the booking a few days before the flight. I think it is when something syncs on to the airport computer system? Anyway, I think it is an internal ba reference. Sorry, I can't explain any better atmorris, I just wanted to show you aren't the only one to notice this and it does exist - it is not a TA reference, the normal mmb PNR, or a us/aa PNR as others have suggested.