Transit/map marker, you see em at boundaries and or high points in different area's, Black Mountain in Kentucky (highest point) has one. USGS USDA(forestry dept), sometimes even local municipal places.
Edit: I've found one as well that was placed there by city GIS/Survey folks
Edit 2: That number on it will more than likely be in hard copy or digital records somewhere, check with the county roads commission etc. DCMatt is on to it too.