Allan H. Stevenson dating history

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Allan Henry Stevenson (June 20, 1903 – March 31, 1970) was an American bibliographer specializing in the study of handmade paper and watermarks who "single-handedly created a new field: the bibliographical analysis of paper." Through his pioneering studies of watermarks, Stevenson solved "the most fascinating, and perhaps the most notorious, bibliographical problem of our time," the dating of the Missale Speciale or Constance Missal, an undated incunable (book printed before 1501) believed by many to pre-date the Gutenberg Bible (c. 1455), and possibly to have been the first printed European book. Stevenson proved that the book in fact had been printed nearly twenty years later, in 1473. Through similar analysis of watermarks, he also established that most block books,
small religious books in which the text and images were printed from a single woodcut block and which many believed dated from the early 15th century, had in fact been printed after 1460.