Until 1931 the Southern Railway initially maintained the locomotive numbers from its constituents, and solved the problem of more than one locomotive having the same number by letter prefixes denoting the main works of the former owning company. All ex-SECR locos were prefixed by "A" (for Ashford), ex-LBSCR by "B" (for Brighton) and ex-LSWR engines by "E" (for Eastleigh). Isle of Wight locomotive numbers were prefixed by "W" (for Wight). New locomotives were prefixed by the letter of the works where they were built. In 1931 the fleet was re-numbered by dropping all prefixes, leaving E-prefixed numbers unchanged, adding 1000 to A-prefixed numbers and 2000 to B-prefixed ones, an exception being the Z-class 0-8-0 shunters whose numbers A950-A957 only lost the prefix, with no addition. (Some non-revenue-earning locos were exempt from this scheme).