Sunset single-sheathed boxcars.

Are you sure you are talking about a recent sunset model?   Any photos?   O r the ebay reference?

From you description, it sounds like an old Max Grey or maybe International Models piece.    

He might be referring to the 3R Sunset Models War Emergency Box Car?

If you search eBay within O scale by that title, you should be able to find it easily enough.

I saw the same model online. it is an older offering, maybe much older.

That era of Sunset freight cars appear to be for made the crossover market of 3R and 2R.  

Sad, because it wouldn't have taken much more cost or effort to do it right a la the Lionel 27xxx scale detail 3 rail cars. 

There are still some glaring gaps in the steam era freight car market such as 1924 ARA single sheathed cars, Southern SU box cars and of course UTLX X3 tankers. 

Hope springs eternal!

I have always hoped that they would do the 700 cars, all in metal, and the only change would be steel insulated wheelsets.

So far, I am the only one . . .