I found records under his name at Ancestry.com the August 13, 1861 muster roll which had been copied (typewritten) May 8, 1933, Armstrong Stewart was not recorded.

Further, the National Archives Compiled Service Records for the Company I, 19th Alabama Infantry, contains only one muster roll for the Company, that being for the period:

for 13” days of Aug. 1861 to 31” day of Oct, 1861 This roll did not record his name.

That results in the loss of some 11 bi-monthly muster rolls in which his enlistment must have been recorded as a post war muster list contains his name.

A typewritten Muster Roll of Captain James H. Savage’s Company “I” “Cherokee Rangers”, Nineteenth Regiment Alabama Infantry records:

Armstrong Stewart, Private, Company I, Cherokee Rangers, died at Chickamauga

This was: Copied from the Cherokee Harmonizer, published at Centre, Alabama, Cherokee County on
Thursday, 5, 1903 Recorded June 2, 1930 Hugh W. Cardon

I have been unable to ID his son William.