Aircraft production terminology

[barnbstormer]

I'm going through some Curtiss Company files, in a company booklet of "Curtiss Wright Airplane Model Designations. One of the description fileds in the production and designation charts that has me stumped is "S.O." Does not seem to fit "Special Order" or "specifications .....??"
as there are already other fields for contract numbers, Model Specification# etc
Not much rhyme or reason to the S.O. column numbers. Most seem to be numbers between 100-530, but veering off into the occassional oddity of "16064" (for the XP-60D Hawk, or even "D-13; D-16; D-18" for the AT-9 Jeep?

Any ideas? I'm sure a WIXER must have the answer..

As in this sample listing for the SB2C-2

Design #84C; Scout Bomber; SB2C-2; 1 built; S.O.100; Navy; Model Spec. SD268-1A; Columbus; Wright R-2600-8; Contract 93691, Marine.

[barnbstormer]

Found the Basic info with some more digging in the files. Came up with some papers with matching numbers as "Sales Order Number." Have not yet cracked the codes as to 100, 200, 400 series numbers yet. But numbers preceded by C, D, & S. appear to be for planes manufactured in St. Louis. The sales order numbers include sales of all kinds, from a sale of one plane to a named individual:
S.O. C-18 4/23/35 to Don Robertson, 1 Curtiss Fledgling
to a foreign Govt, multi-plane order:
S.O. D-1 4/5/38 C.W. export to Bolivia 10 19R trainers
To single plane and multi-plane orders up to 3000, for US Army Air forces