My free opinion;
The number you see is part of the original cast part. It is formed by the mold (likely sand) used to cast the part. The mold is formed by a reusable pattern (made of wood) that was used to make hundreds of brake shoes. There would have been lots and lots of brake shoes with that number cast in place permanently on the side of the part.
Part number: Possibly a number assigned by the manufacturer and built into the pattern used to make the sand mold this was cast with.
Manufacturer number: If you mean a number assigned to each manufacturer (# 1 = Dodge, #107 = Ford, etc.) not likely. Things were not that well organized.
Train Car number: Very very unlikely, Brake shoes were (and are) a low cost commodity, the original builder just slapped some onto whatever car was coming out of the factory on any given day. I doubt they recorded individual "serial numbers" of all the brake shoes installed. And as the cars were maintained while out in the general railroad network the shop would just install the "right size". They did not order a special brake shoe for "Old Boxcar # O 313". They ordered a size 2" x 12" brake shoe (approximate size).
The brake shoe you have could have come off of any of ten thousand (a guess) railroad cars that passed up and down that line "back in the day".
The "O 313" does not seem to indicate a common size, so my guess is that it is a "Part Number" assigned by the brake shoe manufacturer to indicate their version of a "standard" size brake shoe. Standard size brake shoes have existed for a long time. Maybe someone recognizes "O 313" as a code for one of the sizes ?
Cheers, Kevin
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