Alan, I don't have quite enough information to state this categorically, but I have reason to believe that the soldier in question was Martin Luther Butler, born c1844 in Carroll County, Arkansas, who joined the 2nd Arkansas Cavalry, U.S. Volunteers, in 1862, and who had "previously been in the rebel service." I don't know where he was living when the war started, but presumably he lived at or near Fort Smith, since Reid's battery, the "Fort Smith Artillery," was recruited almost exclusively in that area.

If you ever figure out who this Martin Butler is, please let me know. I feel like he's Martin Luther Butler, and I'd like to put him in the "fer sure" category in my database, but I need some more information before I can do that.