Reuben's Company E records consist of only two cards. On the first, he is listed as a Private on his companies March/April 1864 muster roll. This muster was most likely taken on April 31, or very soon thereafter.
The 2nd card shows him as a 3rd Corporal with the troops surrendered to Federal forces by Captain Knox at Citronelle May 4, 1865.
There is no indication that he was previously captured, which I would doubt. Had he been previously captured, we would most likely have found Federal POW records in his file.
Somewhere between these two dates he was advanced to Corporal. Unfortunately, the only company muster roll which survived to be recorded in the Compiled Service Records is one, see below:
Company E
Stationed at College Hill Station, Mississippi, March-April 1864.
April 12.— Company E engaged in the battle of Fort Pillow,
Tennessee at which place Captain Middleton was painfully but not
dangerously wounded in the left arm. Private James L Simmons
was killed on the same day while gallantly storming the fortifica-
tions.