Hello Chris, Good catch I believe you are correct. The confusion for me was on the tombstone at Camp Chase which stated LA. and not L.A. for light artillery.
The following are specifications for Confederate tombstones which would indicate that you were correct.
"SPECIFICATIONS FOR HEADSTONES"
"To be of American white marble, in slabs not less than 39 inches long, 12 inches wide, and a uniform thickness of 4 inches throughout, with bottoms square and at right angles to sides, of fine grain, good texture, and hard; of grades known to the trade as Number 1, Average, and Best Number 2, and in all respects equal in quality to the sample exhibited as standard in this office and in the Depot Quartermaster's Office, at Boston, Mass.; the top of the stone to be sloped from the center to each side with a one-third (1/3) pitch, and the edges slightly rounded; that portion of each stone which will be above ground when set (20 inches from top) to be sand rubbed; each stone to be inscribed with the number of the grave (if required) , the name of the occupant (if known), his rank (if other than a private), the name of the organization to which he belonged, and the letters""C.S.A."" all on one face. If the name is not known, then the word ""Unknown,"" and the letters ""C.S.A."" simply shall be inscribed. The figures and letters composing the inscriptions to be V-sunk, one (1) inch in length and three-sixteenths (3/16) of an inch deep; the letters and figures of the inscriptions to be accurately spaced and aligned, properly and tastefully arranged, and smoothly and carefully cut. Abbreviations may be made in the Christian name of the decedent, and in his rank, as also in the name of his organization, providing that all such abbreviations shall be made in accordance with the list of inscriptions to be furnished by this office. In all cases of abbreviation, and wherever required, proper punctuation shall be observed. In all cases the inscription of the name will be cut on the stone in a curve. The work on the stone to be neat and strictly workman-like in all respects."
All Federal POW Records I've saw just say he was with Beauregard's Battery. However his Company Muster roll do state he was with Captain T. B. Ferguson's Company Light Artillery.
Best of luck with your book! Dennis