24th Missouri, not the 21st.

William A. C. Thompson, Private, Company C, 24th Regiment Missouri Infantry, joined for duty and enrolled/enlisted for 3 years February 1, 1862 at Lebanon, Mo., mustered in March 30, at Rolla, Mo., recorded in June 1862 as “on duty cook,” Decmber 3, 1862, “During the last two months said soldier has been until for duty 60 days. This/Said[?] soldier is subject to fits, having as many as four or five in the course of the ????????? & is also subject to mental derangement at times. Disability ?????? since enlistment,” discharged January 12, 1863 at Van Buren, Mo. on Surgeons Certificate of Disability, Epilpisa & partial Mental derangement, S. N. Baker, Surgeon, description on enlistment: 22 year old Farmer, born in Polk County, Georgia, 5’ 6”, fair complexion, grey eyes, brown hair

The 24th Missouri Infantry was at Rolla, Phelps county, until January 23, 1862, when they took up the line of march from Rolla to the South West on January 23, 1862.

The regiment arrived at Lebanon on the 29th of January 1862, resting a few days at this post where William enlisted on February 1, a few days later they moved towards Springfield remaining until February 12 when they proceed to Rolla where William was formally mustered in on March 30. Here they remained until July 2.

He may have accompanied the regiment from Rolla on January 23 and formally enlisted at Lebanon the next week.

M405: Compiled Service Records of Volunteer Union Soldiers Who Served in Organizations From the State of Missouri.