So far the "It's a hole" seems to win...
Looking through it didn't help - I couldn't see any value in it at all....
When I shift gear with the lever, the adjustment/lockup bolt is visible through it, so I suppose that is as good a use as any. Not that I have any reason to take the lever off - I think that the ability to put the tractor in gear and, - wait for it - CHANGE GEAR is quite useful.
I'll just leave it there, and should anyone ask what it is, I'll reply "It's just a hole".