I have the Walthers car (factory painted C&NW) and yes the paper information the car comes with says it is Thrall built of a type that entered service in the late 1060s. The 19 ribs extend right down to the bottom of the sill (fish belly type bottom, rather than straight from end to end). It has roller bearing trucks.
I am looking at a Train Shed Cyclopedia No 46 that reprints flatcar and gondola information from a 1931 Car Builder's Cyclopedia. There were certainly many 65 foot long mill gons around in that era that would still have been going strong into the 1950s and beyond. Typically such long gondolas were narrower than the 40 to 50 foot gondolas otherwise shown. The Pennsy car G26 class photographed has side ribs but not all of them extend to the bottom of the side as does the Thrall.
Looking at the photos and drawings, there are gondolas painted Central Railroad of New Jersey, Bessemer & Lake Erie, Baltimore & Ohio, Lackawanna 65 foot mill gons somewhat resemble the Thrall in general outline altough the Thrall sides seem rather taller. None of them exactly match the ribs in number or spacing but the visual effect is "sort of close."
One big difference all of those older cars exhibit however, is that the sides and in particular the ribs are riveted, whereas the Thrall is welded. If you are really determined to try to make the Thrall car look older than it is, you may want to explore those rivet decals which I have seen available (but no longer recall the source) and add the lines of rivets. I think MicroMark sells them.
More drastic surgery on the Thrall would be to cut away a foot or so of the top because the cars in the "Cyc" all look a bit shorter than the Thrall. That might be more than you are prepared to take on.
I suspect a real freight car expert and purist (assuming there is a difference!) would call it out as being a hybrid, which of course is exactly what it is, but at the very least you'd be coming as close as possible.
A Car Builder's Cyclopedia from closer to your chosen era might show other 65' cars that come even closer to the late 1960s welded Thrall. But my hunch is that the Thrall sides are taller. Cars built in the 1950s MIGHT have roller bearing trucks. The gons shown in the 1931 "Cyc" reprint seem all to have solid bearing, so called friction bearing, trucks.