The series consists of correspondence files classified as 'Secret' by Navy Office, files with the lesser classification being registered and filed with the general correspondence series. The 'Secret' series was created shortly after the abolition of the Department of the Navy in December 1921 and after the Navy Office central registry moved to Victoria Barracks in June 1922 to come under the control of the Defence central registry.

The use of a multiple number system by Navy Office was recommended by the Royal Commission into Naval Administration in its Report of 1918, but it was only when Navy Office was returned to the Department of Defence that the annual single number system in use under the Department of the Navy was abandoned.
 
The allocation of numbers to primary subject classes was done in alphabetical order, starting at the primary number 1802/-, probably to distinguish the Navy 'Secret' series from the other classified series in the Defence registry.
 
Subject matter classified as secret includes: Air Raid Precautions, Casualties, Chemical Warfare, Censorship, Coastwatching, Court Martial, Defences, Diseases, Expeditionary Forces, Foreign, Guns, Harbours, Inventions, Mines, Naval Aviation, Operations, Plans, Prisoners, Publicity and Propaganda, Reports, Research and Development, Signalling, Statistics, Shipping, Torpedoes, Wireless, etc.
 
Many files in the series have been top-numbered from the previous Navy annual single number classified series (CRS B2887). Similarly, many files from this series can be found top-numbered and attached to papers in the subsequent series.
 
There is also a close relationship with the parallel general correspondence series in use over the period, as files were often registered initially in the secret series and subsequently declassified and re-registered into general correspondence series. The reverse also applied. However, security classifications reviewed and downgraded after the series ended in 1950 remain in the series, which accounts for the considerable numbers of files now bearing unclassified, restricted or confidential classifications.

The files in MP1185/8 are part of Commonwealth Record Series B5573, Secret correspondence files, multiple number series, but have not been physically converted to CRS B5573. The records in this series should be requested under the MP1185/8 number. Other elements of B5573 that have not been physically converted are MP1049/5 and MP1049/9.


Administrative information
 
The following data was keyed from the paper documentation:
Form number:              CA 11
Transferring department:  Defence (Navy Office), Central Registry,
                          Classified and Unclassified
Archives file number:     RWM38/5/1