I recently did some volunteer metal detecting "archeology" at the Plumb House in Waynesboro, Virginia (with permission of course) and all finds were donated to the Plumb House Museum. One of the more interesting finds was this gag swivel for a horse harness. Though the Plumb House witnessed lots of activity during the Battle of Waynesboro, one source informs me this item was not military issue. But that doesn't necessarily mean it's unrelated to the BoW. Soldiers, particularly Confederates, often used their own personal items and improvised when necessary. Of course, it could also be post-war. An interesting artifact recovery, nonetheless.
The item is illustrated in a Pittsburg [sic] Harness Supply Company catalogue below.