There is also a small building in the background of one of the photos. It is a one room building with a fireplace. There is nothing indicating that there were facilities or electricity. There are several of these spread throughout a few acres in Lancaster, CA. What could have been their purpose? I have a few photos of the inside if requested.
Well, it's not so much an actual mailbox as it is a 30lb propane tank repurposed as a mailbox. If you want a more specific date you could try over at /r/whatisthisthing ... but identifying when the tank is from will only give you the oldest date the box could be from.