New to me, maybe not anyone else. Near a bridge with a stream gauge. This probably won't go anywhere until someone takes out the concrete apron.
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Interesting find. I like the hex nut. Does that mean a threaded rod was set first and then the disc was attached? What's the diameter of the disc?
Probably a concrete anchor. Drill a hole, tap the anchor in, tighten the bolt. They are tapered out at the bottom with an expandable collar which expands and grips the concrete.
Honestly don't know for certain.About 2" as a guess. Wasn't part of our scope, just stumbled on it in passing.
It's a fender washer, and you can buy them in a number of sizes. It's a solid little reference mark.
It looks like they deformed the top of the stud to prevent removal of the nut and washer.