Wissenschaftliche Stadtbibliothek Mainz, Hs I 171 is a manuscript from the second half of the 15th century. The provenance is from the Carthusians of Mainz.
The text on ff. 21v-22r is identified as a "Vision de anno 1387." The title identifies it as a vision found in an old book: "Sequitur alia visio Reperta in uno antiquo libro."
The text is in fact the "Cedar of Lebanon" vision (see Robert Lerner's 1983 Powers of Prophecy for the definitive study).