Dear Ms. McDowell,
Thank you for posting your request on History Hub!
According to The History of the A. E. F. by Shipley Thomas (1920), the 26th Ambulance Company was part of 3rd Sanitary Train, 3rd Division. We also searched online and located several examples of APO 740 being used to designate the 3rd Division and its subordinate units.
We searched the National Archives Catalog and located Muster Rolls and Rosters, 11/1/1912 - 12/31/1943 that may include rosters for the company. For more information about these records, please email the National Archives at St. Louis (RL-SL) at firstname.lastname@example.org.
We also located Historical Records of Motor Ambulance Companies, 1917 - 1919; Historical Records of Evacuation Ambulance Companies, 1917 - 1919; Records of Medical Units and Offices of Infantry Divisions, 1917 - 1919; and Records of Divisions, 1917 - 1920 in the Records of the American Expeditionary Forces (World War I) (Record Group 120) that may include unit records of the company and/or the 3rd Sanitary Train. For more information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at email@example.com.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RL-SL and RDT2. We apologize for this inconvenience and appreciate your understanding and patience.
We also suggest that you request a copy of his Official Military Personnel File (OMPF). OMPFs and individual medical reports for enlisted men of the U.S. Army who were separated from the service after October 1912 and prior to 1958 are in the custody of NARA's National Personnel Records Center in St. Louis. In many cases where personnel records were destroyed in the 1973 fire, 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. If there is any information you do not know, you may leave it blank or provide estimates (such as writing World War I for years of service) however the more information you provide, the easier it will be for NPRC staff to locate the correct file. You may also attach a letter or memo with any additional information such as his unit name. For more information see Official Military Personnel Files (OMPF), Archival Records Requests.
Due to the coronavirus public health emergency, the National Personnel Records Center is servicing only urgent requests related to homeless veterans, medical emergencies, and funerals which may be faxed to 314-801-0764. We thank you for your patience and look forward to resuming normal operations when the public health emergency has ended.
We hope this is helpful. Best of luck with your research!