WO 2871904-1949War Office: Confidential Printed Papers (B Papers)
Confidential reports and handbooks printed for internal circulation in the War Office and the army. The original distinction (in the early 1900s) between papers in this series and those in the A series (WO 33) is not clear, but many B papers are military or intelligence reports on foreign countries. Later, B papers appear to be those of a classification below 'secret'.
The B series appear to have been phased out from 1943 and the A series was used thereafter for most accountable documents.