I am looking at the fine work by George Eichelberger titled "Southern Railway Equipment Drawings and Photographs - Volume 1, Book 1 - 40' All-Steel Box Cars" which I really recommend if you are doing detail modeling of Southern box cars. I note this as I am NOT an expert on running boards but only off this to help.
On page 146 reference is made to "steel Morton products longitudinal and latitudinal running boards (roof walks)."
On page 156 there is a detailed drawing of the running boards also referencing Morton as the supplier. On this drawing it shows a close up of the boards showing an arrangement of larger circles surrounded by six smaller circles.
