The Southern Pacific had General American Car Company build 1,200 of these single-sheathed 50' box cars in 1923. This group of cars was numbered from 66800 through 67999, and had single-sheathed ends. They were initially designed to carry automobiles, but later started carrying other bulky loads, such as lumber. A prototype photo and a S-scale drawing of the car can be found in the Mainline Modeler magazine 1983 issues from March and April. Both of these are provided as a link in the kit's instructions.
These are the diagrams from the Mainline Modeler magazine.