Schuyler,
My January 1952 ORER says series 11300-11599 (40' XMR auto boxcars, 12'-6" staggered side doors, 40t capy, with Evans Auto Loader devices) had 292 cars in it. Is this the same series you asked about?
I think have a 1960 ORER buried in a storage box somewhere; I could try finding it tomorrow and check, if you want.
Todd Sullivan