A file of signals intelligence reports, messages, and correspondence issued by the Government Code and Cypher School and sent by the head ('C') of the Secret Intelligence Service (MI6) to the Prime Minister, Winston Churchill. This file includes the following messages: that intelligence based on the analysis of patterns of Japanese naval communications (traffic analysis), not decodes, is to bear the codename THUMB; and extracts from signals sent by Washington to Colombo concerning Japanese naval forces in the Singapore area, based on traffic analysis, for the period of February 20-23.