I am not sure that you are correct Hugo, I also had the same clutch seal mod done on one of my engines by Tony Maughan, the alloy outer ring may be a little thicker than the bonded seal C18/1 that it replaces, the internal O ring is simply compressed to the same thickness as the alloy outer ring when the C20 is tightened up, the C19 is in fact a shim washer, not a seal, the variance in between the thickness of the alloy outer ring and the C18/1 can easily be accounted for by the use of a different thickness of C19 shim, if the C20 was much shorter it would simply lock up the the C17 clutch centre bush. I think Maughan and Sons can still supply the two part clutch seal mod if asked. The real culprit can be the lip on the C17 clutch centre bush, if that lip is too thick you might have to use the very thinest C19 shim to avoid locking the clutch centre up, and that thinest shim is far to thin to be used by itself and can be a recipe for disaster if it breaks up, then the C45 spring tang has a tendency to snap off, the C20 unscrews itself and before you know it you have lost all effective use of your clutch lever, as happened to me in the middle of Dartmoor this summer, I managed to limp back to Gulworthy in 2nd gear, and then of course visited Marcus Bowden, problem solved.