Dear Mr. Rands,
Thank you for posting your request on History Hub!
“SO” in this context normally refers to special orders, as opposed to general orders (GO).
We searched the National Archives Catalog and located Records of Field Artillery Brigades, 1917 - 1919 in the Records of the American Expeditionary Forces (World War I) (Record Group 120) that includes records of the 57th Field Artillery Brigade. We also located Records of Divisions, 1917 - 1920 in Record Group 120 that includes the 32nd Division and files of the 57th Field Artillery Brigade in Entry P 1241-32, box numbers 26 or 27. For more information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at firstname.lastname@example.org.
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We hope this information is helpful. Best of luck with your family research!