I'm thinking this thing started out as a royal cart, and was then changed to be a prison cart.
When I first seen it on a catalog, I was thinking it was a royal cart, too. But when I start to learn English, I understand that it was a prisoner cart and not a royal cart. But I think your theory is good.
I made a royal cart in the same way:
http://www.inorchestral.com/David_Girar ... rriage.htm