Scott, is the "O" a long O or a short O in shitometer. Asking for a friend.It looks like a cast iron shitometer cover with a nearby witness post so that it can be found more easily when the leaf cover is heavy, as is the case in this photo.
When the lid rises above grade, it serves as an indicator that the s%itter is full. Simple and effective technology.
I was there in 2009. I have other photos and even a video of Jessica and I climbing the stairs to the very top of the mansion. A nice view up there.Interesting, I didn't know they had a geothermal heat exchanger loop in the mansion. It looks like it was installed in 2008. In 2020 they put out a bid for a contractor to convert it from an open to a closed loop and make some updates - not sure if that's been completed yet. The bid has some more photos of the system in the exhibits at the end.
It gave me pause that when Scott first mentioned it, that people were giving his post the thumbs up, like "yeah, they are common, and I understand what that is".