Did you notice the other Fonby/Fornby soldier in the same regiment and company?

Fonby, J. T. (also Fornby, J. T.), Pvt. Co. H, 17th La. Inf. Appears on Roll of Prisoners of War, Captured and paroled at Vicksburg, Miss., July 4, 1863.

If your Columbus Franklin had a brother named J.T., then this could be the bit of circumstantial evidence that you need.