You probably will never get an ID on that particular flag. What I am betting is that it was a "Trophy" captured in battle and turned in to higher headquarters to be documented and turned in to the War department. These trophies as well as freind unit colors were sometimes used to decorate sites used for ceremonies, or in this case, a photo shoot.

There is a picture that was taken of some of Sherdian's scouts in either 64 or 65 that had them carrying captured Reb flags also,