Why is the interlocking-squares (gammadia) a symbol of the Melchizedek Priesthood?

I was reading about the relatively new discovery of the Apostle Phillip's gravesite in Turkey, and it briefly mentioned that the square was represented in his grave and it was significant because the square is a symbol of the Melchizedek Priesthood. Mosaics and Images from Byzantine Ravenna, Italy: I was also reading about it here: Christian iconography **Why is the square a symbol of the Melchizedek Priesthood? What ancient documents detail this?** **If no ancient documents, then are there Jewish or early Christian graves that somehow convey that the square represents the Melchizedek Priesthood?**