You didn't find it in Fold3 as it is not filed under it's own name but in Roll 73, [Poindexter's Regiment Cavalry] Q-Y, Preston's Battalion Cavalry

Andrew J. Walker, age 24, Private, Capt. David G. Hicks' Company, Preston's Battalion, Missouri Mounted Volunteers,* enlisted May 11[?], 1862 at Bloomfield for 3 years or the war, value of horse $122, horse equipments $10, last recorded on December 1, 1862 as "hospital here sick" [Pocahentas, Ark.], no further muster rolls

* This company formerly served as Company C, Preston's Battalion, Missouri Cavalry

Early in 1863 Preston's and Burbridge's Battalions Missouri Cavalry were consolidated and formed the 4th Regiment Missouri Cavalry, of which this company became Company C.

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A. J. Walker, Private, Company C, Burbridge's Regiment Missouri Volunteer Cavalry,** enlisted October 15, 186_ at Bloomfield, Missouri by Hale, deserted August 19, 1863, no further records

Note: There is one subsequent company muster roll, that for the period Jan. & Feb., 1864 in which he is not recorded. We can assume therefore that he had been dropped from the rolls. Without further muster rolls we cannot ascertain if he later returned

** This company formerly served as Company C, Preston's Battalion Missouri Cavalry

M322: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Missouri