Paul - It's not quite a build date, but AAR Class "XM" (Boxcar, Steel Underframe), #6001 to #6999 are listed in the January, 1943 Official Register. Maybe someone with an older version can use their Way-Back machine to look further.
Another related find...the Santa Fe Rwy. Historical and Modeling Society Website has an AAR booklet (Rev. 1950) on Rules Regulating The Safe Loading of of Bulk Grain in Closed Cars and Protection of Equipment. Diagrams and photos included! http://www.atsfrr.com/resources/Forms/Pam36-1950.pdf
Another shot of Frisco Box Cars in grain service in 1976. Thanks for the photo Steve. http://www.frisco.org/vb/album.php?albumid=142&pictureid=1726
