Bruce
I have removed the felt from the box and positioned the plywood pieces over their shadows on the felt. I'm pretty sure the plywood belongs there, as the felt has a bit of an impression where it sits. There would have been a piece of plywood at each end of the box. The remnants are there for the left side, and the impression is fairly deep on the right.
First photo is of the left side, with the remains of the end piece, and the second photo is of the right.
I can only see screws on the back, including a couple at the level of the lower plywood. The screws are located where the plywood would have the widest portions of wood, but I would agree that something is missing; perhaps more plywood support inside.
I found an old cotter pin inside the box, but I'm not sure it is related.
Lower box dimensions are 130cm wideX12.5 cm deepX 15cm high.