I have just checked out the Vocab for 6MT4 section, dated 1956 and found the FV switchboards, and all components are listed and available for demand separately. The switchboards listed for the following vehicles;
Thornycroft 6x4, AEC 6x4 and 6x6, Champ, Humber FV1601, FV1100 10ton 6x6, Ferret, Albion/Jones crane, Blaw Know grader, AEC AA tractor, Dennis excavator truck, Scammell Explorer.
Nowhere is there a lock barrel listed for the switch. They might only have been issued on an as required basis for vehicle security in certain circumstances.