The valid possibilities are UA, UN, CA & CN for Union Army/Navy and Confederate Army/Navy, but I have seen NG in this column, and some of these relate to individuals younger than 60 (one was 51) which makes them unlikely to be a Civil War veteran.
Any idea what NG stands for. See Newton township, Calhoun county, Michigan for examples. Unlike the column in 1930 asking which war (CW, SP, WW), 1910 is restricted to the Civil War only.