The drawing doesn't show the off wheel horse, but in the photo, the off wheel horse has a riding saddle too, though no rider obviously. The drawing of the off leader shows no riding saddle and I'd think the off wheel horse would be the same. I'm sure in real life harnesses may have been swapped around when an off harness needed repaired and there was only an extra near one to replace it, and that may be what happened in the photo too.
They also have McClellan saddles rather than the Conestoga type (I think) in the drawing, and the McClellans are covered with black leather rather than the rawhide left bare for cavalry ones. But otherwise it looks quite similar, and may be a match for a slightly different period type.