I'm tempted to say that the car is restricted to one railroad - some sort of maintenance service. The AAR has long had rules about the stenciling and placement of same on cars used in interchange. Cars not used in interchange had very different (and looser) rules, basically road initials and number and in M.O.W service, as Boxcab said - somewhere.
With a little image enhancement, I see there is something that looks like small lettering on the side above each truck. It seems that there is a plate between the two stanchions that is about half again as high as the side frame. There also seems to be something on the bottom frame just to right of center. Pity the image resolution low.
