Nebraska has several manmade national forests where various species of pine trees were brought into the Sandhills region in an attempt to forest parts of the state that basically had nothing else there but hills and grass.
It was a very methodical process with each area having a specific type of tree to see what would grow best. At the corners of these plots were markers. I have seen wood posts scribed with the information, threaded pipes with stamped end caps, and also the more elaborate markers like the one shown below.
The office at Halsey has rolls of maps drawn on linen showing the various tree plots, but I've never seen detailed information that actually describes the monuments. I doubt they are survey accurate, but rather just points placed to mark corners of the various plots.
This is one of the more interesting markers I have found that appears to be polished, but I believe it was an alloy of brass and aluminum to give it this appearance of almost gold looking. The marker was placed in April 1933. It marks the Northwest corner of a 1.6 acre plot for P308P which contains Pinus Ponderosa ... Harney trees. The Harney reference is most likely that the trees came from the Black Hills near Harney Peak.
Off topic, but I'm impressed that your Garmin 12 appears to work with its internal antenna. As of a couple of years ago my 1999 12CX will only track SVs when an external antenna is plugged in. 🙁
> I believe it was an alloy of brass and aluminum
It might be aluminum bronze -- roughly 90% copper and 10% aluminum -- though I don't know why they would have chosen that over bronze (copper/tin) or brass (copper/zinc). Copper is usually the more expensive material in any of these alloys.
I love the Garmin 12. I had three of them and still carry one in the vehicle at all times.