Another piece of evidence for Menkaure, at least, being buried in his pyramid is that there's an inscription next to the entrance that says he was:
"The text is composed of at least five lines in sunken relief. The western part has deteriorated badly and only a few signs are visible, but the eastern half of the text ie, the part which is furthest from the entrance, is in better condition. It gives us the date on which the king was buried in his eternal abode, his tomb, though, unfortunately, the year is missing. The stone has weathered badly in the place where the year ought to be, but from the preserved part we know that his burial took place on the twenty-third day of the fourth month of the winter season. This month begins roughly in the middle of February, and thus we can say that Menkaure was buried in his pyramid in March... ...Four cartouches containing royal names can be distinguished, with some difficulty because they are very badly preserved. The names in two of them are beyond recognition, and a third is not certain. The preserved cartouche contains the name "Menkaure." (The Pyramids, Ahmed Fakhry p.257)
Jon
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