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Dear Mr. Willis,
Thank you for posting your request on History HUb!
We searched the National Archives Catalog and located a series titled Records of Divisions, 1917 - 1920 that contains file units for the 1st Division and the 28th Infantry Regiment in the Records of the American Expeditionary Forces (World War I) (Record Group 120). The War Department Decimal File System is used to determine the 3-digit file codes for the general administrative section within this series. The War Department Historical Decimal Classification Scheme three-digit decimal number indicates the record type, with station lists, unit histories, operations reports, and field orders predominating (see attached). This last manual currently is not available online, which we hope to rectify soon.
In the Records of U.S. Regular Army Mobile Units, 1821-1942 (Record Group 391), there is a series titled Records of the 1st Through 338th and the 559th Infantry Regiment, 1/1/1916 - 12/31/1921 that may include records of the 28th Infantry Regiment. Both of these series are in the custody of the National Archives at College Park - Textual Reference (RDT2) (email: firstname.lastname@example.org).
We also located Special Media series that may be of interest -- 1st Division Operations Maps, 1921 - 1928 and Aerial Photographs of Operations with Interpretations, 1917 - 1919 in RG 120. The maps are in the custody of the National Archives at College Park - Cartographic (RDSC) (email: email@example.com) and the photographs are in the custody of National Archives at College Park - Still Pictures (RDSS) (email: firstname.lastname@example.org)
We hope this information is helpful. Best of luck with your research!