Well, you're eliminating all our options!
Only thing I can think of now could be a little tricky- looking up into the clutch housing while someone slips the clutch. Is this a power director, or direction reverser?
Where is this tractor? There used to be a tractor like this in Auburn NY. I saw it once, even worked on it. But I don't remember if it had PTO, rockshaft, etc., or whether it was a shuttle clutch or not.