A good question indeed. My answer is complete speculation, but we carried our hand tools on U.S. tanks in a canvas bag. That bag is normally kept inside the tank to keep it handy, but the book says it goes in a sponson box outside. So, small hand tools come to mind, perhaps some small petroleum cans? I think I've seen a photo of a knocked out Panther with some sort of tarp or overcoat hanging out of one of those boxes as well. But once again, I'm just guessing.