I think someone fit it into the box or built the box to house the machine. The owner must have felt some need to provide the machine with extra protection and looking at your machine's condition it payed off. I can see a simple packing crate from the manufacturer but dovetails were reserved for permanent containers and drawers. No one would have used them for a packing crate. I've seen that style of bentwood case with the carvings before but I don't remember the manufacturer that used them.
Very pretty machine, it looks brand new and the box is a nice bonus.
Rodney