I agree that it doesn't look like the remnants of a melting. My father melted a lot of lead for reloading. All the pits I saw had very smooth bottoms. The elevation discs I saw (after retirement did surveying for 3years) had a similar hole for the tip of the rod. But some didn't. There would be a square etched in the concrete sidewalk. You had to have the elevation book to have the data (location, elevation, description). Otherwise it looked like a disc that someone stuck in the sidewalk as for how it could get to a field.
If a roads was torn up, the disc could pop off and end up in the dirt. Often times, they have excess dirt and I've seen it sold as fill.
That is a real long shot. But the disc does look similar to the elevation discs I saw. It certainly looks too well formed and symmetrical to be an accident.
