I've been looking into this after I got side-tracked on my original research on the radiotechnology component of the grail myth. There's a bunch of fascinating articles by a Russian scientist and Egyptologist Scariatin "Enel", who undertook work in the 30s and 40s on the Egyptian Mysteries. The links to those papers disappeared some months ago, but I was lucky enough to download them. My take is that the Eleusinian mysteries are a survivor of the Egyptian mysteries, and the Egyptian mysteries themselves came from a much older culture that perhaps originally developed the temple technology. These are the mysteries that Herodotus was initiated into, and of which "he said no more".

The Egyptian version has a fascinating account of pyramid grain, with substantial powers. That's where I clicked on the link to the grail, as otherwise, the concept of a "grail" at least as found in northern European accounts, is absent.