The man who served in Co. D, 3rd Arkansas Cavalry, U.S. Volunteers, was a Yell County man named Thomas Graves. He was born c1827 in Franklin County, Tennessee, and survived long after the war, dying in 1919 at Blaine, Arkansas.

Most of the soldiers (U.S. volunteers and Confederate POWs) who died at St. Louis were interred in what is now the Jefferson Barracks National Cemetery, but there's no Thomas Graves buried there.

You mentioned the enlistments in Perry County. I've noticed that Perry County and Saline County men who lived near the county line often enlisted in companies organized across the county line.