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The originator of this book-test method is Jim Sherman. It was marketed by the National Magic Co. in 1938 (see, for example, their ad in The Sphinx, Vol. 37 No 10, Dec 1938, p. 22). The original used two metal cards with a rectangular window cut into them; one gimmicked, the other for examination. The application of the Wordo principle to a gimmicked paper clip is Bruce Elliott's, and appears in “Fate”, his trick in Phoenix, No. 177, May 6, 1949, p. 708.