According to the Professor, one of the Museum of Dubious Authenticity's field acquisition teams discovered this sign during a recent foray to remote regions in the state of Montana. It was found hanging on a barbed wired fence at an abandoned airstrip in a remote area. The team quickly acquired it (it was almost dark, no one was looking) and transported it to the museum. The sign has been dated as late mid-century, most likely the mid-1960s.
At least, that's the Professor's story. Like so many of his stories, we suspect this actually was made in his secret lair and is destined for a private collector. But it is a good story, isn't it?