I don’t pretend to know anything about Ransomes Dumpers but here are few thoughts that may help to jog your memory.
Does the hole pass all of the way through the item and is it parallel all the way? If so it could be a support for a control rod or similar.
Is the end of the hexagon sealed off or only has a small hole in which case it may be either a breather or a sealing cap?
Is there a thread in the end of the hexagon section that would allow the piece to be a threaded adapter for a pipe to be attached?
Hope this helps, Ray.