[AudiA4B6US]

a dealer can contact Audi to lookup the code if you have the VIN of the car the unit was originally (first) installed in. Some don't like to do this because if the unit is stolen it's causing all kind of trouble. But it is possible...

 

[AudiA4B6US]

I doubt that you can get into engineering mode with the unit locked and no, the serial number is not printed on the label. However, there are some units which had the serial number and/or code on a separate label but that's not original...