After my blog post about modeling a Minneapolis & St. Louis box car appeared (it’s at http://modelingthesp.blogspot.com/2011/12/modeling-project-m-box-car.html ), Douglas Harding sent me a photo of a M&StL car which is much closer to the Train Miniature (TM) model. You can see that it not only has the Howe truss arrangement of side bracing which the TM model has, but also has Z-bar bracing like the TM car.
I don’t know the date of this photo, but the reweigh date on the car is June 1938, so it probably was taken no later than the end of 1940 (reweigh intervals at that time were 30 months).
But there is a drawback to this prototype choice too. There were 212 cars listed in the M&StL number series 18000-18999 (even numbers only) in the July 1946 Official Railway Equipment Register, but by 1949 none remained to be listed. Since I model 1953, that pretty much rules out an 18000-series prototype, but for modelers of earlier eras, this would probably be a better choice. Thanks for the interesting photo, Doug.