HI Courtney, I found the index card below on Familysearch.org. It is from the US Veterans Administration Master Index, 1917-1940. If it is your great grandfather, he was in the 6th Reg. Marines, 82nd Co (I think) from 1917 to 1921. Although I do not see the service number on the card. Hope this is of some help. joan
Dear Ms. Thompson,
Thank you for posting your request on History Hub!
We search the National Archives Catalog and located the series Official Military Personnel Files, 1905-1998 in the Records of the U.S. Marine Corps (Record Group 127) that includes the file unit titled Taff Elmer David -- [Date of Birth] 07/23/1898, [Service Number] 000088366, [Date of Enlistment] 05/18/1917. The contents of this file unit have not been digitized. To request a copy of the file unit, please mail a completed GSA Standard Form 180 to NARA's National Personnel Records Center (NPRC), (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. You also may fax the form to 314-801-9195. 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!