The idea of putting different covers onto duplicate books for a book test is Johann Nepomuk Hofzinser's. Sometime prior to 1867, he created a set of duplicate books with different covers and trim sizes for a routine called “The Word”. See J. N. Hofzinser Zauberkünste by Ottokar Fischer, 1942; translated into English by Richard Hatch as The Magic of J. N. Hofzinser, 1985. “The Word” is explained on p. 69 of the Hatch translation. Also see, J. N. Hofzinser: Non Plus Ultra, Vol. 3, by Magic Christian, English translation, 2021, p. 208.

Theodore Annemann, before Hofzinser's work was documented in German and later in English, reinvented the idea of duplicate books in different covers for a magazine test published in his Book Without a Name, 1931; see "The One-Man 'Genuine' Magazine Test", p. 46.