WW1 British 'BAB Trench Code No.4 May 1917

Rare BAB Trench Code manual No.4 dated May 1917. Handwritten to front cover is ' Officer Comdg RXA A Battery'.
The BAB Code was a compact, officially classified cryptographic codebook used by the British Army during WWI & designed by Lt Col Barrington Butler. Issued to signallers and commanding officers, it allowed troops to quickly encode and transmit tactical messages over vulnerable frontline communication lines without enemy interception.

Code: 15640