Thanks Gil, that Herreshoff engine is one powerful beast, very impressive, and if I am up that way in the future I will have to consider a visit.
Curiosity finally got the best of me and so I took the valve chest cover off as you suggested (and partially the whole valve chest, as it turned out); I was a bit concerned that the whole thing was going to go "boing" and spit out a pile of parts, but proceeded anyway!
Here are photos of what I found - and sure enough there are fully three offset valves, and channels in the cylinder head. Somehow, I don't believe that this is a very efficient design, but there it is. Also oiled the valves and got it all back together again. It now turns a bit smoother and easier. So, mystery solved (sort of)!
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