I have two 'C' Comets and three Burman boxes, one kept as a spare. The spare one has the threaded and slotted adjuster. This was used on other makes with Burman boxes. As you say on the Comet it is a pig to get at and adjust in situ, hence, I suppose, Vincent's use of the un-threaded ferrule. The non-threaded Vincent Part No. is G94/1SS Comet Clutch Abutment.[/QUOTE
Vincent chose to provide adjustment by cable so the threads in the inner cover serve no purpose on the Comet and Meteor. Hugo,even more of a pig for access is the Comet Valve Lifter cable; that can be a real struggle!