In answer to three of your questions.

What does F&S stand for? Field & Staff.
What does A.C.S stand for? Assistant Commissary of Subsistence. The position of regimental A.C.S. was abolished by the Confederate Congress on May 1, 1863 with the general order being published May 29, 1863.
How did you become a Captain? During the American Civil War, one became a captain by either appointment, promotion, or election depending on the position.

I hope this helps.

Respectfully,

Gerald D. Hodge, Jr.
M.A. Military History - Civil War Concentration
Research - Preservation
Historian: 39th Georgia Volunteer Infantry Regiment