ADV 397X is a suffix style registration number and is our own stock number and therefore you will not find it cheaper or get it transferred quicker anywhere. It is currently held on a retention certificate which means the transfer could either be completed online onto your vehicle immediately, or it will take 5 - 7 working days for the DVLA to assign the registration to your vehicle. This registration can be held on a Retention Certificate until you are ready to transfer it onto your vehicle.
This style of number is tied to the date in which it was initially released. Created as part of the 'X' release, this was in August 1981 and so your vehicle must have been registered on or after this date. You can make a vehicle look older than it is by putting a registration on which was released before your vehicle was manufactured, but you cannot make a vehicle look newer than it is by putting on a later registration.