The cards are blue, pink, white and lemon and lined. According  to a report written by J.K. Jensen, 18/1/1910, on the Defence Department card system, the blue cards are subject index cards. He states - "As all proper names must be included in this (the
alphabetical) index it will be understood that "Fremantle Barracks" is also contained therein. This would apparently mean that "Fremantle Barracks" is indexed in 2 places, viz.: in the subject index and under "Fremantle" in the alphabetical index. In practice, however, the "Fremantle Barracks" card (blue) is taken out of the subject index and placed behind the word "Fremantle" in the alphabetical index. All subject cards relating to "Barracks and Buildings", "Drill Halls", "Lands", "Rifle Ranges", "Vessels" etc., are therefore found in the alphabetical index. This obviates opening two cardsfor the one subject and when the card relating to "Fremantle
Barracks" .... is required, immediate reference is made to
"Fremantle".
Many cards are marked at the top with "Secret". Each is headed by a range of letters, e.g. "As - AZ" or by a name, e.g. "Brig. Gen. T.A. Blamey", Underneath in columns folios/files are
indexed in chronological order: first the date of indexing
is recorded; then the file title and often with a precis of
the particular folio indexed; and then its registration.
The registrations are mostly of complete files rather than of folios. The registrations relate mostly to the classified
3-number series used 1906-1935 for security classified correspondence. This series used primary numbers ranging from 1802
to 2029. However, there are a few entries relating to general correspondence, especially the annual single number system used 1901 - 1906 and the multiple number system with primary numbers ranging 401-664 used 1917 - 1929. The entries to these general correspondence series were made presumably for the purpose of cross indexing - some cards are marked at the top with "cross index". Interspersed among the cards are indicators showing
ranges of letters.
 

Subsequent series unregistered

Nominal index cards to secret correspondence files, multiple number series (class 401,) 1936-1944 held by Army Archives