Dave, thanks for posting the cool pic! I don't think off-hand that I've seen a photo of it in service, excepting I guess the photo in the RGS story. It's kind of a funny looking car since the truss rods are so close to the body...looks like they might have been over-tightened too.
Well, looks like for now we have to assume the mystery car is the 051. I can't for the life of me figure out why they would use that in such a consist, but, I guess it was available, and maybe on short notice, no other baggage cars were.
Also, if you find anything out more about if any of it survives, please let me know. I've seen the photos in the pictorial of the car in the stockyard in 1955, and assumed it had survived till later, but, geez, if it's still around, that's quite a find. Amazing how many of the early cars lasted so late into the 20th century. There are still a few gems out there, including some of the B&S boxcars, that still haven't been preserved.