If it was originally some sort of "grab box" inside a vehicle then, clearly, it wouldn't be bolted down.
I have seen metal "grab boxes" for bren mags in a WW2 Canadian vehicle. (I am NOT inferring that the box pictured in post 1 is for bren mags before someone tries to say that I am.)
I would have expected the hinge to be "piano type/style" hinge running across the full width of the box but this may have been replaced.