There's nothing behind that hole but the void. It gets pretty rusty in there but I don't know what you can do about it. The surfaces of the tubes are tinned when the ribs are layed, so that helps. Although you can't measure that area with a gauge the walls in that area are generally a little thicker as a result of being subject to less filing when the barrels were struck.
Bill if that is the same hole I am thinking you are referring to it is just a threaded hole that had a short screw in it. The metal part that the screw is in is actually an early reinforcing rib and is attached to the for-end loop. It is almost like a set screw and is flush with the rib. It does not hold anything and I would not worry about it.