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Have you requested his Official Military Personnel Files from the National Personnel Records Center? That might be a great place to start if that’s available. If that’s not available, proof of service can be provided from other records such as morning reports, payrolls, and military orders, and a certificate of military service will be issued. Please complete a GSA Standard Form 180 and mail it to NARA's National Personnel Records Center, (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002.
You also may be interested in his unit records. While unit records generally do not include detailed information about each soldier, they can be useful for understanding the activities of the unit as a whole. We searched the National Archives Catalog and located a series titled Records of Divisions, 1917 - 1920 in the Records of the American Expeditionary Forces (World War I) (Record Group 120). We also located Records of Infantry Brigade, 1917 - 1919 in Record Group 120, as well as Records of the 1st Through 338th and the 559th Infantry Regiment, 1916-1921 in Records of U.S. Regular Army Mobile Units (Record Group 391).
I hope this is helpful in your research!
Ben Lester served with Co D, 141st Infantry Regiment, 36th Division from May 1918 till his death at the Battle of St. Etienne on October 8, 1918. This battle is the 36th's entry into combat in WWI.
Information on the 36th in WWI from our website: TEXAS NATIONAL GUARD IN WORLD WAR I (texasmilitaryforcesmuseum.org)
The 141st is part of the 71st Brigade.
Most Company photographs were taken after combat was over during the Division's stay in France in 1919. There is a Souvenir Booklet from Camp Bowie with some photographs but it was done before May 1918 when your relative joined.
If you'd like a higher qaulity scan of these documents in their original from ( in color) let me know.
Texas Military Forces Museum
This website https://wjh.us/AEF/ contains an image of the Burial Card of your family member as well as links that show where he was first interred in a battlefield cemetery. You can also see a list of his Co. D fellow soldiers who were killed that day and buried alongside him.
He would have had a Burial Case File that contained information about his death, the burial card referenced above, initial burial, and perhaps correspondence to and from the family related to his death and burials. Families were given the option of final burial in an overseas National Cemetery or the body being repatriated for a stateside burial.
You can request his Burial Case File from the National Archives. It is contained in Record Group 92, NAID 595318.
Record Group 92, NAID 12007376, contains maps that record the names and initial burial locations of soldiers. These are battlefield survey maps drawn to show latitude, longitude and salient features (roads, geographical landmarks) that would allow the Army Quartermaster Corps to locate the soldiers for later reburial. I've seen Burial Case Files that contain that plat, however you can request it separately.
I noticed that his uncle was listed as next of kin. I don't know if that was an indicator that his mother was deceased. Gold Star mothers and widows were provided escorted trips from 1930-1933 to visit the European grave of their son or husband. The National Archives holds these records