ABSTRACT
Although the source codenamed MASK came to an end in 1937, the British intercept operators maintained their vigilance and in 1940 they were rewarded with access to a new source, codenamed ISCOT. This traffic continued, unread, until the end of 1942, but initially MI5 had no clue as to who was handling the messages in London. The first clue materialised in February 1943 when James Shields, a member of the CPGB's feared Control Commission, was seen meeting Jean Jefferson, a graduate of the Wilson School whom he tried to recruit to run a wireless from her home in Wimbledon.
