UREKA, I've found the answer
From April 1971 on, thicker linings, together with thicker fulcrum pads and the necessary longer rear brake cam lever with newly introduced return spring was fitted.
The new longer lever was installed 'upwards' instead of underslung as previously.
There is no recorded engine number change point for these alterations to restore braking efficiency, I would guess around DE20000 onwards.......but this is only a GUESS. Roy
