The Southern Railway was formed by the 1923 grouping of the earlier private railway companies. The new group, principally operating routes to the south of London and along the south coast, combined three major companies, the South Eastern & Chatham (SECR), London, Brighton & South Coast (LBSCR) and the London South Western (LSWR), together with a number of smaller railways. The new company had to devise some method of combining the locomotive fleet numbering systems used by the three companies and initially locomotives were identified by adding a letter showing the works responsible for that locomotive. SECR locos received the letter A for Ashford, while LBSCR B for Brighton and LSWR E for Eastleigh. This provided a quick way to eliminate duplicated locomotive numbers while the company reviewed locomotive policy.
Eventually a new scheme was devised and applied from 1931 where the SECR loco fleet received a prefix number 1, so from the early 1930s this locomotive A593 would have carried the number 1593. In many cases the works letter was retained, as locomotives were re-allocated away from their original owners' areas it was found useful to be able to qucikly identify which works should be contacted when spares or works repairs were needed.
The SR 1931 scheme may well have inspired the BR 1948 numbering system, were former SR engines were renumbered into a national numering scheme simply by adding 30000 the existing number. Adding 40000 to LMS loco numbers and 60000 to LNER engine numbers removed any chance of accidental duplications.