Sorry, I used ~51.8667 as an easier way to type 51° 52'. As a1call noted, this slope angle is extremely close to the "golden" (φ Phi based) pyramid inclination of 51° 50' and the Pi-based pyramid inclination of 51° 51'. Looking at http://www.guardians.net/egypt/gp1.htm which has good official diagrams of the passageways in the Great Pyramid, as Warren says the subterranean passage is shown in the bedrock, so this idea of an imaginary upside down pyramid is almost certainly without foundation. And yet, it seems to have a symbolic presence, for example in this picture of thoth balancing interlinked pyramids on his palm, which admittedly is more like a Star of David than an octahedron and comes from a modern woo site not from an ancient source. Ah well, Newton found some of this stuff interesting.