Very interesting. I have tried to read it all but found my mind wandering. I think the "on" picture is the staged image judging only on the fact that it is such a tight grouping of balls ending in the near vission. That would be remarkable cannon fire in those days. I would not be suprised that no balls are on the road in the "off" picture, as the camber of the road most likely with the residual engery of the balls, make them group in the drainage gutter of the road.
Assuming they were fired from the right of the picture.
Unless the road had already been cleared. What if fired from the left, then why are so few on the road?
A hard one.