Look at the Branchline shake-the-box (Yardmaster) '37 AAR boxcar-it has a single
stringer along each side of the center sill.
From: Tim O'Connor <firstname.lastname@example.org>
The AAR 1940 Car Construction recommendation was two on each side
of the center sill. But prior to that I just don't know, and AFAIK
none of the 1937 AAR box car models has gotten it right (I mean, I
can't confirm if any of them is correct).