Block A formed part of the Government Code and Cipher School at Bletchley Park and together with Block B (Monument HOB UID 1525305) was the first purpose built brick building for office accommodation on the site. Construction was completed in August 1942. It originally housed the Naval Section dealing with non-Enigma cryptography in Western Europe and Air Section which included the Meteorological Section which provided weather forecasts for Coastal Command and to predict the movements of U-boats and shipping, and SALAU a section dealing with intelligence on German bomber and reconnaissance aircraft. By mid 1943 the Naval Section had taken over all the building and the Air Section had moved to Block F (Monument HOB UID 1525325).
Block A is a two storey, steel-framed building with pre-cast concrete floors and roofs, and brick walls. It is E-shaped on plan comprising an east-west range with three ranges projecting forward to the south.
After departure of the Government Code and Cipher School in 1946 Block A, together with Block B, was used as a National Service Hostel by the Ministry of Labour. By 1948 it was part of the General Post Office regional training school and by 1953 it was used by Bletchley Teacher Training College. In 1977 the building had been taken over by the Civil Aviation Authority (CAA). Following the departure of the CAA from the site in 1993 the building became the responsibility of Bletchley Park Trust and in 2004 was refurbished as new exhibition space for the Bletchley Park National Codes Centre Museum.