24th Va Cav shows a William B. Smith, Captain of Co K and Robert C. Smith as 2nd Lt, Co K

7th SC Cav had two Sergeant Smith's but no higher rank

Don't know about the Hampton Legion.

I think I would go with the 24th VA Cav unless something else comes up.

24th Regiment, Virginia Cavalry
24th Cavalry Regiment was organized in June, 1864, by consolidating eight companies of the 42nd Virginia Cavalry and two companies of Dearing's Confederate Cavalry. This unit served in General Gary's Brigade, Army of Northern Virginia, and fought in various conflicts around Richmond. Later it was involved in the Appomattox Campaign and surrendered with 19 officers and 144 men. Its commanders were Colonel William T. Robins, Lieutenant Colonel Theodore G. Barham, and Major John R. Robertson.