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. From conversations picked up between the two, MI5 ascertained that the CPGB had recently lost radio contact with Moscow, and was unable to read much of the traffic being broadcast, some of which Shields believed should be relayed to the United States. The third person involved in this effort to re-establish contact with Moscow was Bob Stewart, and MI5 monitored a meeting he held with two other Wilson School graduates, Sam Cohen and his wife, who had previously worked as CPGB wireless operators. When none of these efforts appeared to have borne fruit, and Stewart seemed to spend more time on overt Party activities, MI5’s concentration on the CPGB faltered, but a discovery by the 21st Army Group in Belgium in 1945 offered a considerable breakthrough.
