When the SR was created in 1923, it immediately set about finding a solution to the problem that many of its inherited locomotives carried the same numbers.
Inherited locomotives and those transferred to the island by the SR were renumbered into a single list commencing at 1 and prefixed by the letter 'W' (for Wight).
If the UIC classification was applied entirely correctly it would result in confusion on locomotives with trailing axles, since these are represented by a number.
From 1 January 1879 the principal LSWR works at Nine Elms (and, subsequently, Eastleigh) used an order number series that ran from A1 to Y1, then A2 to Y2, A3 to Y3 and so on (the letter I, J, Q, U, W and Z were omitted).
The system was inconsistently applied with regard to the LSWR A12 class, for which the first batch was allocated the work number 'A12' in 1887, the second in 1888 was 'E1' and the third the following year was 'M2'.