Special Air Service. The SAS traces its origins to 1941 and WW2. It is has been a regiment of the British Army since 31 May 1950, currently forming part of the United Kingdom Special Forces (UKSF). A number of Auxiliers joined SAS after the threat of an invasion was no longer imminent.
Special Duties Branch. Often called Special Duties Section and abbreviated SDS, the organisation formed Part of GHQ Home Forces Auxiliary Units and is always referred to as Special Duties Branch in original documents. (Not to be confused with the Administrative and Special Duties Branches of the RAF.) The Special Duties Branch consisted of civilians, both men and women, who were tasked with the gathering of intelligence by spying on and observing enemy formations and troop movements. They would leave their reports in dead letter drops. The Special Duties Branch also set up a wireless network intended to be used for the passing of information. (See also ATS and Auxiliary Units Signals).
Special Duties Section - see SDB.
Secret Intelligence Service. Commonly known as MI6 (Military Intelligence, Section 6), the agency supplies the government with foreign intelligence. It operates under the formal direction of the Joint Intelligence Committee (JIC) alongside the internal Security Service (MI5), the Government Communications Headquarters (GCHQ) and the Defence Intelligence (DI).
Special Operations Executive. Officially formed by the Minister of Economic Warfare, Hugh Dalton, on 22 July 1940, to conduct espionage, sabotage and reconnaissance in occupied Europe against the Axis powers, and to aid local resistance movements. It was initially also involved in the formation of Auxiliary Units.