Todd, I believe the drop dead date for conforming to the USSA hardware requirements was 1916.
That's about the time that all the remaining inherited cars, for the most part the Peninsular 30' box, coal and flat cars, were stricken from the roster, to avoid the expense of conversion. But many of these cars were converted to work equipment, not subject to the requirement, as they were not used in commerce.
Jim Courtney
Poulsbo, WA