Postal Express Service - motorcycle courier or not?

Question

I'm attempting to research my grandfather's military history during World War I. He worked for the Postal Express Service, and I'm attempting to understand if he was a motorcycle courier or rather what work he would have conducted at each of the sites he was stationed at. 
 He was in the South Dakota 1st Cavalry initially and then transferred to the 136th, then 127th and then the 116th. 
 - Would a sergeant at this time have written a motorcycle? - Given his rank and movements, what would have been his role in the army throughout his career. - Would you have any suggestions for other areas to research? 
 --------------------------- Signed up May 31 1917 - enlisted July 15 1917 
 Appointed Corporal Quartermaster April 26 1918 
 Sergeant Quartermaster August 31 1918 
 Maybe Sergeant Jan 26 1919 
 Sgt Motor Dispatch Service May 31 1919 
 Sgt 1cl PES Co D 116 Sup Tn 
 Out of service July 1 1919 --------------------------------------------------- 
 South Dakota 1st Cavalry 
 136th Inf - he was also assigned to VORC (what was VORC?) 
 Transferred to 127th MG Bn or Machine Gun Battalion 
 Transferred to 116th Supply Train, when he was appointed Corp QMC 
 Went to 1st Depot Div HQ 
 went to replacement depot 
 He was in postal express service, in Paris and Chaumont, and appeared to move around every month or so. This movement was one reason I was wondering if he was a courier.

Textual Reference Branch, Archives II · Answer

John Cass 
 Thank you for posting your question on History Hub! 
 Initially, we would recommend that you request your grandfather&rsquo;s Official Military Personnel File (OMPF) . OMPFs and individual medical reports for soldiers of the U.S. Army who were separated from the service after October 1912 and before 1960 are in the custody of NARA's National Personnel Records Center (NPRC) 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. Veterans and next of kin of deceased veterans also may use eVetRecs to request records. See eVetRecs Help for instructions. If there is any information requested by the form that you do not know, you may omit it or provide estimates (such as for dates), but the more information you provide, the easier it will be to locate the correct file if it survived the fire. For more information see: https://www.archives.gov/personnel-records-center/military-personnel/ompf-archival-requests . 
 If you are interested in performing research on the various units he was assigned to there are a multitude of series that may be of interest to you. Initially, within Record Group 391 Records of U.S. Regular Army Mobile Units there are the series Records of Cavalry Regiments, 1916&ndash;1937 (which includes records of the 1st Cavalry) as well as the series Records of the 1st Through 338th and the 559th Infantry Regiment, 1916-1921 which holds records of the 137th Infantry Regiment. In addition to RG 391, Record Group 120 Records of the American Expeditionary Forces (World War I) holds the series Records of Divisions, ca. 1918&ndash;1942 which has records of the 137th Infantry Regiment, 35th Division in boxes 12-13. This series also has records of other various smaller units such as Machine Gun Battalions and Supply Trains. RG 120 also holds the series Records of Machine Gun Battalions, 1917&ndash;1920 and Records of Supply Trains, 1917 - 1919 . All of these units can be searched within the Record Group 165 Records of the War Department General and Special Staffs, 1860 - 1952 series Records Relating to the History of the War Department, 1918 - 1941 for additional records. For more information about these non-digitized records, please contact the National Archives at College Park - Textual Reference (RR2R) via email at archives2reference@nara.gov . 
 We searched online and located Army Paper Work: A Short, Concise Treatise on Paper Work for Army Officers. Giving Forms and Explaining Their Use and Preparation and General Orders, United States. War Department, 1917 that defines VORC as &ldquo;Verbal Orders, Regimental Commander.&rdquo; Perhaps this means he was assigned via a &ldquo;VORC.&rdquo; 
 We invite you to continue the conversation with community members on History Hub, but should you have follow up questions for the staff at Archives II, please email us at archives2reference@nara.gov so that we can assist you further. 
 We hope this assists you with your research! 
 Sincerely, 
 Textual Reference Archives II Branch (RR2RR) 
 [RR2RR 24-67259-VVT]