Yep, as mentioned above, there are fewer canonballs in the ditch in the photograph where the road is clear. That would indicate either that it was taken first because of fewer total cannonballs...or that it was second because the "recycling" involved the balls on the road and some from the ditch.
Additionally, the additional balls on the side of the hill in the "clear road shot" could indicate that it's the second photograph and that those balls were among those not gone after in the recycling.
But it's also possible for them to be up on the side of the hill in the alleged first photo (with fewer total balls) but absent in the alleged second (with more balls) if the additional balls on the road and in the ditch were relocated into the frame from outside of its scope.
If the balls were recycled, the question is then, by whom? It's inferred that if the van was being drawn by army mules on that day there were army muledrivers. Fenton doesn't mention them. It seems likely that had he been in the company of military personnel, his letters would mention this.
On the other hand, would he have ventured into the vally of the shadow unaccompanied?
Which version of the events is consitant with the facts? If there was a barrage taking place after Fenton set his tripod, from which he retreated 100 yards, let's suppose he exposed a plate at the time he set his tripod. It's entirely reasonable that had he waited out a barrage for an hour and a half and at that time the area was more littered with shot than before, he would have exposed a second plate.
In my opinion, Fenton's letter's notwithstanding there is something else about these photographs that bothers me. I see two, differing arrangements of cannonballs on a roadway but in neither do I see the evidence I would expect to see of cannonfire. I see no craters, no toen ground and no tracks from rolling, tumbing or boincing balls.
What I think I see are canonballs spilled, say from a peviously overturned wagon. Or perhaps, spilled by Fenton or at his direction to entirely stage a photograph that still wouldn't look like the result of an artillary barrage. I find that possiblity surpassingly unlikely.
Fenton's letters aside, I present the possibility that he accompanied a party that either set out to salvage a previously lost load of cannonballs or happened upon them and decided to salvage them.