Alan,
I have never heard or seen anything about a 933W box. A little bit of conjecture here, there is a listing for a 633F which is the G. Fox boxcar. There also was a red 633 and a tuscan 633 reefer along with a 633 brown and white B&O box. Too many variations of the 633 number. Perhaps the thought was that when the 900 series knuckle coupler was introduced a letter would be assigned to identify the various colors. There was only one version of the 933 produced though which was the white and brown B&O. Perhaps the 933W box was the only version printed until someone came to their senses. Just a thought.
Rich
