The idea of secretly slipping the case flap between the top card and the rest of the deck appears in “Improved Penetrating Card” in Glenn Gravatt's Encyclopedia of Self Working Card Tricks, 1936, p. 394; and later as “The Penetrating Cards” in Jean Hugard's Encyclopedia of Card Tricks, 1937, p. 338.

• Related category in Denis Behr's "Conjuring Archive"