At most, this was a stop on a long gone rail line. You can see the prior railbed melting back into the desert immediately east (half-a-hurl) from highway 127 on google maps. (Lat 36.14671, Long -116.31802) It is unclear that there were ever any (many?) buildings here.
Not sure why the circle was put there, but it's continued existence can be explained by this statement from a wiki:
"Currently, there are no residents of Evelyn. No structures remain. All that remains is a paved loop of road next to the old railroad grade. The California Department of Transportation uses this area to store paving materials and test equipment for work on California State Route 127 and other area roads."
I suppose if the state highway maintenance guys get bored, they might run their paving machines in head-to-head "tether car" competition, but they're gonna need a BIG post....
I find the "marker" there just as interesting. It appears to be very close to where I would estimate the corner of the Greenwater Range federal lands to be. Although I don't seem to be able to navigate the USGS database for benchmarks....to confirm that. It doesn't immediately strike me as an earthquake fiducial marker, but trying to find a listing/map of those is elusive, too.
I'd be very disappointed to find it was just a key lockbox for paving equipment being staged there....