After the Battle of Atlanta (July, 1864) WHT Walker's Division was broken up and Mercer's GA Brigade set to replace Lucius Polk's Brigade of Cleburne's Division (whose regiments were pretty much splattered over the rest of the division).
Mercer's Brigade were carrying at the time battle flags from the Charleston Depot. In early October, 1864, they received their own set of Hardee pattern battleflags as did those untis of Cleburne's Division that lost flags in the Atlanta Campaign. The 8th Mississiooi should have been issued one of these flags.
Based on the surviving flags of this issue, the flags reverted to the circle center from the rectilinear center of the Spring, 1864 issues. The unit designation may have been painted within the circle.