Thanks for the information regarding the "Reith Knapsack", you may want to type "reith knapsack" in Google search, it should come up with the following book: "The Confederate Army 1861 - 65 (4) Virginia and Arkansas." Page 21 will give you details on this issue.
The location of William's wound or its severity is unknown but he was transferred directly to a prisoner of war camp, possibly indicating the wound had broken no bones and that it was a flesh wound of no great concern to the Federals having made no record of it.