I don’t know anything about your ancestor’s company and regiment, but after looking at the info found in his compiled service record, I see where he is shown as being captured on 2 Oct 1863 and eventually being sent to Camp Morton in Indianapolis where he died as a prisoner of war on 2 Mar 1864. From my own personal researching experiences, I have found many times that dates are approximate only and may not always be 100% accurate. Even though the former 2nd lieutenant wrote what he remembered happening to your ancestor many years later, I would believe his first person account, with the exception that the event he described may have actually taken place in early October of 1863 instead of September. Then again the date of his capture shown in his compiled service record may be inaccurate and it may have actually taken place in the latter part of September. You definitely know he was captured though, instead of being killed. His compiled service record confirms this along with his 2nd lieutenant’s description of the event in his post-war testimony for your great-great grandmother’s pension application. By the way, what are the names of the 2nd lieutenant and your great-great grandmother (as shown on her pension application)?