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The image has clearly been adapted from John R. Neill's illustration for the copyright page of L. Frank Baum's Dorothy and the Wizard in Oz. Gibson began producing cards
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I find it fascinating that someone went to the trouble to reproduce Neill's Scarecrow drawing - including the club he's holding and the emerald he's sitting on! - rather than coming up with a more generic image. I wonder if it was an intentional Oz reference, or simply drawn by a fan of the books. The artist is unknown, as were many of the Gibson postcard artists. I'm not aware of any other Gibson cards using Oz images in this way - many of their cards were single designs, and this doesn't seem to be part of a series.
The company was a latecomer to the postcard craze of the early 20th century, and certainly did not commit themselves to the concept. Gibson cards were not produced in great quantity, and were not marketed nationwide, so they are among the scarcer and more desirable Halloween postcards.