COOK & BROTHER ATHENS, GEORGIA RIFLE. - James D. Julia, Auctioneers
Lot 1016. COOK & BROTHER ATHENS, GEORGIA RIFLE. (31289)www.morphyauctions.com
Due to the way the letters are offset I am thinking it reads:
T N
ATHENS
It could be a commemorative notation by a Union soldier captured it near Athens TN, although I am unaware of any major battles or skirmishes in that area.