Don Valentine asked:
"Speaking of early 50 ft. cars I'll bring this type up at least one of which ended up here in Vermont on our little St. Johnsbury & Lamoille County RR. This was a 50 ft., single sheathed ex-Santa Fe car that had the heaviest cast underframe I can ever recall seeing. It is that underframe that makes it stick in my mind because there were hollowed out places within it to account for the pivot of the trucks on sharp curves. I have no idea when it was constructed or by whom and, unfortunately, never got a photo of the one that sat just east of the Eastern States Farmers Exchange building for some years in the late 1970's and early 1980's. Does anyone know more about it from this meager description?"
StJ&LC also had secondhand 50 ft SS automobile boxcars. These had fishbelly sidesill reinforcements at the door opening, but are definitely not the same as the massive underframes on the ex-ATSF automobile boxcars. I believe you conflated the features of the two series of cars.
My gut feeling is that these are ex-CB&Q, but I'll leave it to their experts on the list to confirm.