No, the negative is not reversed. Schiffers is too good of a researcher to fall for that and you aren't giving me any credit either.
The photo show a group of Infantry Officers, three seated, one standing. Also standing is an Infantry Sergeant with his rifle at the order on his right side. So is his cap box.
Of the three holsters visible, two are being worn on the right side, angled to the front, butt to the rear. One of the seated Officers appears to be wearing the issue holster on the right, butt facing forward.
Two of the holsters, while both flap type, do not appear to be large enough for the 1860 Army, so I suspect that they were smaller, privately purchased arms.
Sounds more like the negatives were reversed.
They weren't like the 'Mexican Loop' holster that could be worn on either side - they featured a 'dedicated' right-hand-only flap and loop.
Look more carefully at the photos.Scouts Out!