There is only one prototype I think we uncovered that one of the MTL models is close to, is the Bethlehem Steel 14-panel straight-side drop-end car. The prototype has 14 identical panels, whereas the MTL model has wider 1st and 14th panels than panels 2-13. But it's close enough that at least the MTL model can pass, after it is extensively lowered.
All of the MTL steel straight side gondola models are foobs anyway, as they simply are fishbelly sides with straightened lower sills and adjusted ribs. The rivets slope down and disappear where the fishbelly was, as new rivets never were tooled for the center straight side section. Same thing with the 50' flatcars. So I don't believe they would be suitable for kitbashing 42' or 45' cars with extensive modification.