Good question, l could find no primary source, for when it was first discovered in 1968 during debris clearance of the north face.
Its often quoted that it contains Menkaure's name and mentions that he died on the 23rd day of the 4th month of summer (l.E.S. Edwards)
R. Gundacker did a modern study of it, in Sokar 19, 2009, but l couldn't find a copy. Hopefully more info in that article.