I believe the 56th Cavalry Regiment, Partisan Rangers were participating in the Georgia campaign at the time of the Tupelo affair.

No. 73--(756) Mentioned by Col. E. McCook (Union), Atlanta campaign. No. 74--(646, et seq.) In Ferguson's brigade, Jackson's division, Gen. L. Polk's army, Atlanta campaign. No. 75--(456) Mentioned by Colonel McCook (Union), June 11, 1864, as at Ackworth and Dallas.

No. 78--(857) Lieut.-Col. William Martin, Colonel Boyles commanding Ferguson's brigade, Wheeler's corps, September 20, 1864.
http://www.civilwarhome.com/56alacav.htm