Here are some photos of things I found in or near the mystery circle. None of these is necessarily original to the site, but I suspect they are from nearby.
The first is what I assumed to be a piece of the turntable pit wall, but which could have been something else entirely. My ugly foot is for size comparison.
Same chunk without my foot and from a different angle. It appeared to be stones mortared together, which is what I understood the pit wall to be made of.
A few pieces of coal and what looked like mortar from between bricks or stones.
There were a few pieces of wood laying around. This one caught my eye.