British Army Low Grade ciphers used in World War II
British Army low grade ciphers were intended to be used by the headquarters of all units and formations, not possessing a common higher grade cipher, for tactical messages dealing with operations, movements and administrative plans likely to be effective in the near future. These messages dealing with plans or operations scheduled to take effect after more than 48 hours were not to be sent by wireless unless a medium or high grade cipher was available.
On 5th November 1943, the Army Council introduced Double Transposition as the only low grade cipher. It used a pair of number keys to transpose letters of a message. Each message would begin and end with a disguised key indicator consisting of four numerals.
Due to the complicated and error-prone nature of Double Transposition, a new low grade cipher was authorised by the Army Council on 3rd February 1945. This new cipher was called LINEX and used a grid of random alphabets to encode a message. The LINEX key setting of four letters was placed at the start and end of a message. However, the key setting was disguised first by encipherment using a Playfair Square.
For messages that didn't need to be totally enciphered other systems like Map Reference and Slidex could be used from time to time.
There was also another code introduced in 17th June 1944 specifically for reporting states and returns, e.g., tank, workshop, and bed states and ammunition, POL, and supply returns.