This paper presents an edition with translation and commentary of an extended text that was inscribed on a plate (or conceivably a pair of plates) that lay against the rear face of the Antikythera Mechanism while it was under the sea. This plate, which may have functioned as a protective cover, is extant only in small fragments, but more of its text was preserved as offsets on a layer of accreted matter that built up against it. The text was a systematic description of the dials, pointers, and other external features of the Mechanism, beginning with the front face and continuing with the rear face. The best preserved passages include descriptions of features on lost parts of the Mechanism: a display of pointers bearing small spheres representing the Sun and planets on the front dial, and a dial on the upper back face representing a 76-year “Kallippic” calendrical cycle.