Bret:
Sources indicate that the Macon Arsenal and the Noble Brothers Foundry being the possible source of the heavy 12 pound guns is not possible. Information taken from "Confederate Cannon Foundries" by Larry Daniel and Riley Gunther, published in 1977 by Pioneer Press, Union City Tn, indicates that the Noble Brothers began cannon production in April, 1861 of types smaller than the 12 pound heavy gun, the type you seek info on. Their entire production of 58 cannon was of the 6 pound field gun, 3" rifle, and the 12 pound howitzer types, all of a field artillery type. The heavy 12 pound gun was not intended for field operation but for fixed installions This firm did produce a few heavy guns but after April 1862, of a 8" seige gun size, too large to be the guns you seek. They ended all cannon casting in November, 1862, in effect being fired by the Confederate Goverment.
The possibility that the guns were cast by the Macon Arsenal is even more remote in that Macon delivered their first cannon in March, 1863 and their entire war-time production of 72 cannon were Napoleons and Parrott types only. None were of the heavy 12 pound type.
The best possibility remains that they were cast by Tredagar in Richmond VA, in 1861. This foundry did cast cannon of the type you seek while Macon and Noble Brothers foundries did not. Again, they were of the "heavy" 12 pound bronze field gun, type 1841 pattern. Hope this helps.
Regards
Ron