I don't find him in MS CS Graves Registrations.
I don't find him in the 7th MS in MS CS Pension Applications, but I do find him in the 26th MS therein.
Isaiah Barnett, 26th MS, 1906, Lee County.
Lula Barnett, 1918, Lee County; said her husband Isaiah Barnett was in the 26th MS Infantry.
In Howell, I don't find him in the 7th MS.
However, in Howell, I do find Isaiah Barnett, Pvt., Co. I (Capt. Burton's Co., raised in Itawamba, Pontotoc, and Tishomingo Counties, MS), 26th MS Infantry.
I also find Isaiah Barnett as a member of the Boomerangs, i.e., Capt. Taylor's Co., a local defense company (but county of origin not specified).
Hope this helps! -- Jim Huffman, Gainesville Vols