The flag of the 18th Georgia Battalion taken at Saylor's Creek is in Savannah, GA in oprivate hands. It is of a pattern unique to GA units from that area and two other flags of the pattern survive - one in private hands (56th GA) and the other at the Atlanta History Center (5th GA Cavalry).
The 18th GA Battalion flag at the MOC (WD 430), would have replaced the flag taken at Saylor's Creek.
Regarding the 12th Georgia Artillery battalion, the MOC was one, WD 359, also taken at Saylor's Creek by the 1st CT Cavalry. The Georgia Capitol Museum has a flag from this unit as well, taken at Ft. Stedman by the 3rd Maryland Infantry Battalion, WD 263