'Maintenance' Serials
Distinct serial numbering systems are used to identify non-flying airframes used for ground training. The RAF have used a numeric sequence with an 'M' suffix sometimes referred to as the 'Maintenance' series. Known allocations, made between 1921 and 2000, ranged from 540M to 9344M, when this sequence was terminated. The main series of single letter serials did not use 'M' to avoid confusion with the suffix 'M'. The Fleet Air Arm use an 'A'-prefixed sequence (e.g. A2606) and the Army Air Corps issue 'TAD' numbers to their instructional airframes (e.g. TAD015).
Read more about this topic: United Kingdom Military Aircraft Serials