Chevvron - the McCulloch engines had much finer cylinder fins closer spaced - I had a close look at the Wallis Autogyro engine - the man himself showed me over it after a display. It was also fitted with a downdraught carb on top of the crankcase. The carb in fact was a Claudel Hobson AI 48 HIM from a Gypsy Major.
Dont ask how the hell I remembered that as I could not tell you!