If the flange is left over from deep wading trunks, that would suggest that the box is attached below the stowage tray. If you look along the top edge of the box, it appears there are footman loops, as were seen along the edge of the tray to strap down items on the tray. That could mean that we are looking at the top of the metal box facing outward, unless they wanted to raise the stowage tray every time they needed access to the box.
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