Here is a photo link from the Barriger Library:
Use the arrow pointing down at the lower right corner to see a larger version of the photo.
This undated photo was taken on the Missouri Pacific.
Notice the distinctive car in the middle of the photo. This appears to be a car that was in captive service and didn't leave the MP. The car lacks initials and a road number but does have a sign that begins to read "For Exclusive Use..."
I'm sure some of you already know all about this or similar cars so please share what you know.