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 . During the Second World War RAF aircraft carrying secret equipment or that were in themselves secret had "/G" added to the end of the serial, the "G" signifying "Guard", denoting that the aircraft was to have an armed guard at all times while on the ground, e.g., "LZ548/G"—the prototype de Havilland Vampire jet fighter, or "ML926/G"—a de Havilland Mosquito XVI experimentally fitted with H2S radar. Prior to this two-letter, three-number scheme, the RAF and preceding Royal Flying Corps (RFC) used a serial with a letter followed by four numbers, e.g., "D8096" - a Bristol F.2 Fighter currently owned by the Shuttleworth Collection, or "K5054" - the prototype Supermarine Spitfire. The aircraft serial number does not change.