In the UK Royal Air Force ( RAF ) the individual serial takes the form of two letters followed by three digits , e.g. , BT308-the prototype Avro Lancaster , or XS903-an English Electric Lightning F. 6 at one time based at RAF Binbrook .
In the case of the UK Royal Air Force ( RAF ) the `` Serial '' takes the form of two letters followed by three numbers , e.g. , BT308-the prototype Avro Lancaster , or XS903-an English Electric Lightning F. 6 at one time based at RAF Binbrook .