thanks Mark,
since the back doesn't show any similarity with a 600s, I wonder:
was this just a thin board glued into a tiny recess of the guitar's top or something more 'in deep'?
Was this one of those one off experiments or a fairly regular custom order?
THe tops of the old guitars were a 2" wide piece /the center section with the maple sides glued to that .
This sandwich was routed from the back,a crude X brace (sometimes 4 indiviual pieces) glued into the body and then the 1/4" thick back was glued onto that .
The ledge cut for the binding and you have a body.
Of course this is a shortened version for this posting ,but I'm sure you get the general idea.
As a side note ....many backs are indeed plywood not solid maple .And some of them HAVE a seam in the center !