Inscription reads:

SIXTEENTH INFANTRY
CAPT. ELI W. BOTSFORD,
MAJ. MILTON MILLS,
2d BRIG. 4th DIV. 13th CORPS.

photo from Ohio At Vicksburg by William P. Gault
and graciously provided by Larry Stevens - Ohio in the Civil War

Major Milton Mills was sent here shortly after his capture, along with other officers of the 16th O.V.I., at the battle of Chickasaw Bluffs, December 29, 1862, and remained for about five months before being exchanged at City Point, Virginia, in May of 1863.
photo credit: National Archives