I know nothing about it.. but I am confident from the top and bottom photos you are likely correct about the "solid piece" theory... and all I have to say about that is "WOW"! I don't know if I have ever seen anything like that. what piece of wood would they have gotten that from, and how was it cut out not to cross the center of the tree (making it likely to crack),, !?!?
Id almost recommend you email it to someone like the the PBS show "new Yankee workshop" and see if he has a comment about it.
regards ,Ed