From experience of cataloguing thousands of ACF drawings as well as seeing a substantial number of Pullman-Standard drawings, it was typical that drawings used to manufacture parts or assemblies received a lot number (or similar term each builder used for compiling their order list). Several thousand ACF drawings were created with the purpose of developing a new design, but if the design didn’t materialize with actual hardware the drawings received no lot number. Still other drawings were developed at the request of a customer for purposes of making a cost estimate, but these drawings were not assigned lot numbers if a production order didn’t follow. Similarly “blank” lot number on ACF drawings occurred when an order originally had drawings annotated with a lot number and in some cases production had begun, only to be erased if when the order was canceled. 

Numerous freight car diagrams reference what I believe Standard Steel Car Co. called an Office Order (O.O.) number, a term that Greenville Steel Car Co. & Pressed Steel Car Co. also used to document orders received. While I suppose anything is possible, it would surprise me if SSC drawings used to manufacture production parts or assemblies did not reference an O.O. number. If not, surely SSC used another method to cross-reference orders & the drawings that applied. 

For the years I’m most familiar with (1923 to early 1930s), SSC order numbers were typically chronologically-assigned using a separately numbering system for each plant the cars were assigned to be built. These plant locations included Butler, Pa., Hammond, Ind., and others SSC acquired over time. Some SSC order numbers on freight car diagrams are denoted with a letter prefix. The prefix may have always been part of the official SSC order list, however, I’ve seen many references to SSC O.O. numbers that didn’t include a prefix letter. Prefix letters “E”, “F” & "G” followed by 4 numerical digits commonly used for freight cars known to be built at Butler in the 1920s to 1931. Prefix letter “H” followed by 3 numerical digits are referenced on some Rock Island diagrams built at Hammond, while NKP diagrams had Hammond-built cars assigned a 5-digit number without any letter prefix.

In 2002 Eric Neubauer published a spiral-bound booklet “Pullman-Standard Freight Car Production, Including Predecessors.” Starting on page 130 it lists SSC production 1902 to the early 1930s. Included are O.O. numbers for those Eric knew at the time.  In the 18 years subsequent to that publication I’m reasonably sure that Eric has since located additional order numbers. 

Regards,

Ed Hawkins