I am pretty certain this old painted metal flag on wooden post would have been used as a hand-held railroad signal flag, with the black and white meaning stop, the yellow meaning caution, and the  fishtail shape distinguishing it as a "distant signal," providing advance warning of stop or caution ahead. It seems to be quite a rare one--I can really find no equivalent examples out there--and I think just terrific looking, on both sides, with the creamy white stripe against the black and then this super rich, beautifully aged yellow. The post is black-painted wood with a slit running most of its length into which the metal flag is inserted, and  with what I believe is its original, utilitarian hook on the yellow side.

Overall dimensions: 24" t x 18" w. Flag itself measures 12 1/8" t x 18" w. Great, I assume early vintage condition, structurally super sound and sturdy, with some surface stratches and a little paint loss around the edges of the flag, as there should be. Paint is very stable (not actively chipping), and beautiful!