[motto]

I believe that what you have there are some of the side boards used on the DUKW Amphibious Truck when carrying stretchers. They clamp to the upper edge of the cargo compartment coaming with the board to the outside. With the stretchers cross ways they (the stretchers) rest on the upper edge of the coaming and these boards prevent them from moving to left or right and dropping into the cargo compartment which would presumably have another layer of stretchers lengthways on the floor.
When not in use they are stowed in the left hand side underfloor area between front and rear wheel arches.

David