I spotted (placed into the industry) two cars, built specifically as stock cars, on Conrail, in 1990. This was at Oak Island Yard, at a location named "Stock", where the connector from the LVRR to the PC took effect, just west of the roundhouse. This was obviously well past 1980. Stock was named for the cattle unloading operation, and abatoir (sp?) at that location. The cars stunk, and the cows were covered in feces, and probably hadn't been washed, or watered, in days............
Those cars were bi-level, and that was the last time I personally saw a loaded stock car, anywhere. We were called on duty, specifically to spot those cars, when they came in, from the west.
Having also pulled the Blue, and Red Ringling Bros trains, I wouldn't really classify the animal cars, as stock cars. Stock cars typically have open sides, covered in grates, or perforated siding, allowing ventilation of the cars, as well as ability to hose down the animals, without opening the doors. Cars I have seen have sprinklers inside, that you can connect to a fire hose type apparatus, to wash down the interior, and critters at same time. Regards