OK, the skew plane comment got me to thinking.... There is no reason
they couldn't have made a skew plane if they wanted one. It's just a
question of did they have ever wanted one bad enough to go to the
trouble of making it.
So the question I have now is, what specific kind of planes can we
document? For this purpose I'd accept as documentation a) The thing
itself. b) A non-amiguous illustration of the thing. c) A wooden
artifact that appears to be made using the thing.
A good group project might be to start a database of plane types, good
examples and the type and location of the evidence.