If anybody's interested I saw that Eastern surplus has one of these boxes removed from an M934
https://www.easternsurplus.net/PartDetails/2128/Expansible-Expandable-Van-Body-5-Ton. If you do consider buying one inspect it thoroughly in the following areas for rust
Where the roof folds over on the sides and is riveted, if there was debris laying there and it got wet it may have rested holes through all over the place.
The edges of the drip rail on top and underneath.
When you expand the sides and before you open the roof and floor inspect the edges underneath near the roof.
Open the heater and air conditioner access hatches and inspect for rust on the structure areas inside there.
Look for water stains on the wall panels, using a Philips screwdriver remove the screws and remove the wall panel and inspect for rust and dampness behind the fiberglass insulation.
Actuate all the moving components to check for correct actuation and out of square.
This can't be stressed enough Make sure you carefully follow the opening and closing instructions completely or you will damage the front and back folding end walls and/or the side roof panels.
Mine had previous damage from people not knowing how to set it up properly and doing things in the wrong order.