The newest issue (June, 2008) of The OO Road (the publication of the NMRA American OO SIG) contains an article by Edward Morlok on the Hawk OO gondola, with information on the unusual wood sided prototype (it is a sulfur car) and a photo of a nicely built up car.
The Hawk Model Company was located at 3521 Fullerton Ave. in Chicago. A producer of model airplanes, they advertised their line of OO gauge freight cars little. I have never seen a catalog and at present (2008) only have this one complete kit for the gondola, a nearly complete caboose kit, parts for an automobile boxcar that could be rebuilt, and a few extra frames. The most recognizable feature of the cars is the die casting for the frame. Note also the original glue and paint which is in a glass vial with a cork stopper. These were included in the kit which sold in 1941 for 50 cents. They also sold O gauge kits that sold for $1.
No. 500 Gondola (wood)
No. 501 Box car (outside braced)
No. 502 Steel caboose
No. 503 Auto car ( 50’ double door box car, steel)
No. 504 Flat car
Hawk also produced a tank car in OO and I have been told that they also produced a stock car but the latter is unconfirmed by other sources. If you have more information on Hawk OO, especially the catalog they list in a later advertisement, I would welcome hearing from you. My contact info is in my profile.
UPDATE: I note that the draftsman of the plan for the gondola is P. L. Mates. The cover photo of the April, 1941 issue of Model Railroader is of a Philip Mates of Chicago working on an O gauge interurban, there is a good chance it is the same person.
UPDATE 2010: I now have a few more examples of Hawk and have updated the links above. Also as this is the general article on Hawk OO, a few more general notes. The company itself dates back to 1928 and they produced a wide range of products. One of their best known products for me was their line of "Weird-Ohs" as my brother made several--a nice profile on the company and the production of these models may be found here. According to the Wikipedia Hawk was purchased by Testors in the 1970s.
UPDATE 2012: And, as noted in a couple of the articles on the cars they made, the boxcars apparently were produced with either a body built up from parts (floor/roof/sides/ends) or a body made from a solid block of wood. Examples in my collection match each other visually perfectly and would appear to have been cut from the same cutters to produce the same design of car.