With your new set of photos added, it is looking more and more like a possible bending brake "hold down" should. I now see the gap, showing it does open, and what looks like 1 inch acme all thread screw hold downs, with wing nuts, that could clamp it down flat. The only thing missing is the bending plate itself, which may have been a couple of pieces of angle iron clamped together. Or, maybe the bending plate, (with piano hinge), was also set into this huge hold down clamp, and sandwiched together with the metal to be bent?. A bending brake requires a solid and true edge hold down, and a straight flat plate to force the work to bend. Did you find anything like that laying around? PS: I have never seen a brake with a separate bending plate before, but i have also never seen a brake 14 ft. long either, so who knows!