This is cool! That led me to reading the Wikipedia page on Survey markers[0], and thence to both the USGS National Geodetic Survey Data Explorer[1] and scaredycatfilms' NGS Benchmark Viewer[2]. On both sites I was able to find some markers not far from where I work, and I'm making plans to check them out.
[0] https://en.wikipedia.org/wiki/Survey_marker
[1] https://geodesy.noaa.gov/NGSDataExplorer/
[2] http://benchmarks.scaredycatfilms.com/index.php