Caxton Watermark QA #11, from Sotheby, Samuel L. Principia Typographica: An Extensively Illustrated Work in Three Volumes, Imperial Quarto, on the Block-Books, Xylographic Delineations of Scripture History Issued in Holland, Flanders, and Germany During the Fifteenth Century: on Their Connexion with the Origin of Printing and on the Character of Water-Marks in the Paper of the Period . 3 vols. London: 1858. (Volume 3: "Paper-Marks")
NOTE: in the Plate, this mark has the number "12," but so does the mark below it. There is no number "11" on the Plate, so the first "12" is here presented as "11."