The number of unknown men buried in that one-time trench has been discussed in this forum as 265 (Cozzens citing Capt Wiles) and at 263 (Jones, “Confederate Cemeteries and Monuments in Mississippi,” pg 105, from Publications of the Mississippi Historical Society, Vol VIII,, 1904) which is repeated on the memorial plaque that's on site at Shady Grove cemetery.
According to O.R.- SERIES I - VOL XVII/1 [S# 24] SEPTEMBER 19, 1862:
"Engagement at Iuka, Miss. No. 2. -- Reports of Maj. Gen. William S. Rosecrans, U.S. Army, commanding Army of the Mississippi, with congratulatory orders. HEADQUARTERS ARMY OF THE MISSISSIPPI, THIRD DIVISION, DISTRICT OF WEST TENNESSEE, Corinth, Miss., September 29, 1862.
… The enemy left his dead on the field, part of them gathered for interment, and his badly wounded in the hospital at Iuka.
His loss was: Killed, 265 … Among his killed were General Little and Colonel Stanton. ...”
If one were to subtract those two "known" officers from this report's total, (and understanding that Gen. Little is not buried at Iuka), one would be left with a count of 263 unidentified Confederates dead on the field -- which is the same count that the marker at Shady Grove enumerates and the Mississippi Historical Society's record.
Also, Kenneth, I've been compiling some lists for you that I'd like to send to you for your review. So far, these lists represent 52 names, who look to be among those previously unidentified Confederates. Email me directly: chip (at) manganholcomb (dot) com