It would appear by his two CSR files that it was indeed the 6th Cavalry. Or perhaps some unit of the Georgia Reserves as noted in the War Department note.

George Martin

Samuel M. Ralston

Residence was not listed; 36 years old.
Enlisted as a Captain (date unknown).
* Wounded 5/10/1864 (place not stated) (Wounded in left forearm)
* Hospitalized 5/19/1864 Atlanta, GA (Institute Hospital)
* Furloughed 7/29/1864 Atlanta, GA (Institute Hospital)
"D" Co. GA 6th Cavalry
[Civil War Research Database]

A note in his file from the War Records Office of the War Department, Washington, DC dated June 19, 1896 records: "It appears that Capt. Samuel M. Ralston, who had served as of Co. D Smith Legion Ga. Cav and Co. D, 6 Regt. Ga. Cav., was reported in Jan '65 as Col. comdg. Regt. of Ga. Reserves (See Office Records Series I, Pat I, Page 963, Vol. 49) [CSR Samuel M. Ralston 6th Cavalry and 6th Battalion, Cavalry (State Guards)]
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Samuel M. Ralston, age 37, enrolled in Captain Ralston's Company, Smith's Legion, Georgia Partisan Rangers** May 16, 1862 at Ellijay?, La

**This organization was one of the companies of the Cavalry Battalion, Smith's Legion, Georgia Volunteers, which battalion became the 6th Regiment Georgia Cavalry in March 1863, this company becoming Company D of that regiment.
[Compiled Service Record Samuel M. Ralston]