I believe there was a Mk-1 with a green muff that had more and finer fins and was more of a tapered shape, larger at the top, of course. This was somewhere just prior to the Mk-2, I believe.

I also think I remember the plain bearing version 2.5cc having a red muff. Perhaps they used those in the early Mk-1's. Just a guess, not even sure if the bore was the same.

These are memories of a young teen with his nose almost against the engine display case at my LHS in the nineteen fifties, so they may be a bit inaccurate.

George