Thanks Bill - here is a little more information if you don't have it. In 1950, the X6004 came as part of a single set, the set # was 1461S. The car came in a box marked "6004" (no X) . Set 1461S consisted of a 6110 locomotive with smoke and magnetraction, 6001T non whistle tender, 6002 black gondola, X6004 box car, 6007 SP style caboose. It came with 8 curved, 3 straight, 1 uncoupling track and a 1012 35 watt transformer. My collectors reference comments that this is a scarce set because it contained (like the X6004) several 1 year components.
Again thanks for your comment
Don