I was in conversation with Waynee Stebbe and it is the concrete lid on the old shaft. I didn't realize it at the time when I was there, or I would have paid more attention. As it was, the bugs were distracting me, LOL. There was a concrete support in behind the lid (in the picture but hard to tell amidst the vegetation) which would have been the support base for a structure that was above the shaft, I imagine. It has been years since the place was closed up, so not surprising that there is little left visible today.

NL