There's a very good chance this is an error.
This class was built as A-50-16 (A for Automobile), and apparently carried that class until disposition, unlike for instance the first RBL cars on UP. The class did get renumbered in 1955 from 474000-474499 to the 175000-175499 series.
The B-50-21 (B for Box) class, 184000-185899, was essentially the 1937 AAR 10'0" high car with square corner 5-4 ends, but with some special UP touches like the route card holder being located on the bolster tab instead of the door, and some other less visible ones. They did not have ACR sides though the A-50-16 did.
Above from 'Union Pacific Freight Cars, 1936-1951' by Terry Metcalfe. My copy is bloodstained from the model railroad accident I had on July 4, not sure of the year but around 1995. In the evening I was doing touchup paint on something at my workbench, and I used a large pair of pliers to try to open a bottle of Floquil paint that had gotten stuck shut with dried paint. The lid did not yield, but instead the bottle broke, the boxcar red (not UP oxide red) got all over my hands along with the blood, so actually it may be a mix of blood and paint on the book. When my wife took me to the ER, the nurses tried several times to use alcohol and any cleaning agent they had to wipe the sticky red stains off my hands and kept being frustrated and not understanding why that red stain wouldn't come off. Let's just say I missed the fireworks displays that evening.