Very thorough explanation, Neil ! I don't believe I have ever seen that word before (unless I saw the Welsh version and thought it was a typo, lol) -- but your description of it as half a walnut shell makes me think I have seen them on our "reality" TV show "The Amazing Race," where the contestants had to use them to get themselves across a little river (although I have no recollection of where they were at the time). They appeared quite difficult to maneuver!
