REPORTING CODES - an outline.
Reporting Codes were first used by the Great Western in 1934. Their purpose was to identify long distance express trains. The first number of the code identified where the train originated, the second and third character identified the train.
In the 1950's the Western Region adopted the codes and they continued in use even when 4 character headcodes were introduced. The lamp arrangement denoted the first character of the 4 character code, the last 3 characters were carried on the frame.
The actual numbers carried varied over the years. To be sure you have the right one find a dated photograph and use that number.