Came across this old post while researching 6 Spring Clutches. Bit late for Dave, but maybe of interest.
For Plunger bikes with their own design of clutch.....
Collet size noted appears correct, as does outer diameter of the thrust washer.
Service sheet 702 gives Plunger Mainshaft Gearbox Pinion Sleeve bore as 0.812", a gnats under 21mm, so centre hole in the correct thrust washer (67-3276/7/8)* matches this. B & M are given as 0.87", getting on for the 22mm mentioned.
C10 and Cll bikes with 4 speed box and 3 spring clutch use a similar washer, same OD but a smaller centre holes to match 3/4" mainshaft.
Swarfy.
*Different thicknesses available to assist primary chain alignment.