A subject files series was introduced during the First World War and allied with a numbering system to designate each subject (FB 1-2364) from the early 1950s onwards. Original arrangement by number has been retained in this catalogue. In order to give a unique numeric references to files ca 1915-ca 1950, the later subject number and a small case roman numeral have been added. Thereafter, FB references are recorded as found except where they are incorrect or missing. In which case they have been emended. Several files sub-series, opened in the 1990s, have reused existing numbers for completely different subjects, resulting in partial reference duplication.