MarkD should have the records as to exactly what that chip is (assuming it's one of his
) yours was manufactured two years before cats became a legal requirement in this country... it could have been at a time where bmw were flip-flopping between installing them and not.
It might be worth checking to see if there's any O2 sensor wiring been cut back/hiding under the car to see if someones done a professional de-cat on the vehicle. There seems to be a lot of that happening around the uk - people decatting bimmers with the far fetched hope that it'll increase performance.
I recently worked on a 540i/6 that had been professionally de-catted - however the owner could get away with it because the vehicle was LPG converted - ran with an almost perfect emissions score as well. (Was just prone to trying to blow your face off when coming on-idle after high RPMS, anti-blowback valves for the win.)
What you should do I suppose is take your proof of ownership into a bmw dealer and ask if it was born with a cat or not, that'll mop that one right up. It's just then a case of figuring out what the chip is that you've got.
Even if the ECU numbers match, yours could have been slightly adjusted for no O2 or something like that.
Cheers,
Andy