The round tray has been seen on a Dragon MkII, this towed the 2 pounder, far too big for any sighting mechanism.
There is a school of thought that it may be a cable laying device, most artillery and mortar batteries had phone connections, and on the Windsor mortar carriers there are cable reels.
The tube appears to have an attachment for a drum to play out the line, and the other end has a strip to keep the line from jumping out.
If any one has a copy of Moving The Guns, some-thing similar is shown on page 71.
Any one interested can contact me off line at
george.dfs@virgin.net.
Thanks,
George.