Hi Ellis,
AHA!!!!
THe way I understood it is the thrust washer was added to SRM's shaft adaptor to take advantage of the later design where the spring pressure is not taken by the lip on the inner drum, and adding a further friction area for the innermost plain plate
If theres a lip on the inner drum then its a plain plate first and the clutch is working to the original design
and not like the triumph and A65 setup where theres no lip on the inner drum
(then whats the point of the thrust washer??)
In my post linked earlier I machined out the centre of a friction plate so it would fit over the lip and filed the chainwheel slots deeper so the friction plate sits against its inner face
John