Looking at the US Citizen PM files. Gardner sent numerous notes and letters to James O Broadhead in St. Louis. Most in 1863. Typical stuff but interestingly he just signs them as OAA Gardner sometimes with Provost or Provost Marshall appended. I haven't seen a rank prefix on any of them.... almost like he's a civilian in the position. Iowa Adjutant General records list him as Private. FWIW.