The following is transcribed from Battles and Leaders of the Civil War Grant-Lee Edition, Volume II, Part II:
The Finding of Lee's Lost Order
By Silas Colgrove, Brevet Brigadier General, U.S.V.
In reply to your request for the particulars of the finding of General Lee's lost dispatch, "Special Orders No. 191," and the manner which it reached General McClellan, I beg to leave to submit the following account:
The Twelfth Army Corps arrived at Frederick, Maryland, about noon on the 13th of September, 1862. The 27th Indiana Volunteers, of which I was colonel at the date, belonged to the Third Brigade, First Division of that corps.
We stacked arms on the same ground that had been occupied by General D. H. Hill's division the evening before.
Withing a very few minutes after halting, the order was brought to me by First Sergeant John Bloss and Private B. W. Mitchell, of Company F, 27th Indiana Volunteers, who stated that it was found by Private Mitchell near where they had stacked arms. When I received the order it was wrapped around three cigars, and Private Mitchell stated that it was in that condition when I found him. {See below}
General A. S. Williams was in command of our division. I immediately took the order to his headquarters, and delivered it to Colonel S. E. Pittman, General William's adjutant-general.
The order was signed by Colonel Chilton, General Lee's adjutant-general, and the signature was at once recognized by Colonel Pittman, who served with Colonel Chilton at Detroit, Michigan, before the war, and was acquainted with his handwriting. it was at once taken to General McClellan's headquarters by Colonel Pittman. It was a general order giving directions for the movement of General Lee's entire army, designating the route and objective point of each corps. Within an hour after finding the dispatch, General McClellan's whole army was on the move, and the enemy were overtaken the next day, the 14th, at South Mountain, and the battle of the name was fought. During the night of the 14th General Lee's army fell toward the Potomac River, General McClellan following the next day. On the 16th they were overtaken again, and the battle of Antietam was fought mainly on the 17th. General D. H. Hill says in his article in the May "Century," that the battle of South Mountain was fought in order to give General Lee time to move his trains, which were then parked in the neighborhood of Boonsboro'. It is evident from General Lee's movements from the time he left Frederick City, that he intended to recross the Potomac without hazarding a battle in Maryland, and had it not been for the finding of this lost order, the battle of South Mountain, and probably that of Antietam, would not have been fought.
For confirmation of the above statements in regard to the finding of the dispatch, you are respectfully referred to Colonel Samuel E. Pittman, of Detroit, Michigan, and Captain John M. Bloss, of Muncie, Indiana.