Southern Region units are excluded from this page, as they perpetuated the four-digit numbering series commenced by the Southern Railway - see SR Multiple Unit Numbering and Classification. Prior to the introduction of TOPS, electric multiple unit numbers were allocated by British Rail regions in their own series. Diesel multiple units did not carry unit numbers at first, but the Scottish and Western Regions later adopted their own series for these units, whilst elsewhere individual depots allocated unit numbers, often prefixed by their two-letter TOPS depot code (e.g. TS for Tyseley). When TOPS numbers were applied to multiple units, the unit numbers were prefixed by the class number to make the new six-digit unit numbers. These regional series have been perpetuated since TOPS because it eases the description of individual units and avoids potential confusion if there is only one unit with a particular identifier working in a certain area. Diesel Multiple Units London Midland Region There was no regional number series for DMUs: individual depots had their own series, prefixed by their two-letter TOPS depot code. The table below shows the series used by Tyseley depot (in the Western Region until 9 September 1963). Scottish Region The table below shows the broad arrangement of numbers, though a couple of Class 104 units took numbers in the 3xx series. Class 107 units were later renumbered twice, first to 7xx following introduction of the Class 156 with 4xx unit numbers in the Regional Railways Sprinter series, and then to 0xx following introduction of the Class 158 with 7xx unit numbers in the same series. Western Region Unit numbers were prefixed by a letter (letters prior to 1970) to indicate which area (or depot) the unit was allocated to. The table below lists the prefixes used; Tyseley had transferred to the London Midland Region by the time the 1971 changes took place so was not included, though it did shorten its prefix to TS. The unit number ranges used were: Before 1970 unit numbers were not generally painted on the ends of the units, but at some depots were carried on boards visible through the driver's cab windows. Also, each depot used its own number range within the overall scheme outlined above, so that there could be (and was) a Bristol-based Cross-Country set BL516 and a Cardiff-based set CDF516 at the same time! Inter-depot transfers meant frequent renumbering of sets, so that