ANSWER
It is a hobo sign. Hobos in the early 20th century had a whole system of symbols worked out whereby they could let their fellow travelers know what to expect. They would leave these signs on a fencepost, railroad trestle, side of a building, etc. This particular sign means that the man who lives here has a gun.
WHO GOT IT RIGHT: Marika Thiessen, Bob Milligan, and Jerome Brown.
