Any idea what the view through the 6-pounder (long- Mark I) and (short- Mark IV) gunsight looked like? I don't think open sights were used, and if was a telescopic device I would think both would have a crosshair and/or some reticules, or some other way to adjust for range. I'm also assuming both sights would be slightly different as the ballistic characteristics imposed by a shortened barrel would not be the same.
