Some more light...
The Great Pyramid of Menkaure at Giza
by Alan Winston
http://www.touregypt.net/featurestories/menkaurep.htm
"...In the antechamber, Vyse unearthed the remains of an anthropoid wooden coffin with, Menkaure's name Within were human bones. Most scholars today believe this coffin was inserted, perhaps in an effort of restoration, into the pyramid during the Saite period late in Egypt's ancient history. However, the bone fragments were even more recent as revealed by radio carbon dating, that shows [t]hat they probably date to the Coptic Christian period of some two thousand years ago. There is a rectangular indention in the west section of the antechamber floor, suggesting that a sarcophagus may have once been intended for this room."