What if [P] is not ‘P’ as we use it.
Consider these possible interpretations
A keystone:
Key = Chi = X
P = Rho = 17th letter of the Greek alphabet
Rho reversed = Ohr = ore (stone?)
The CHIRHO representing a keystone?
What does that image remind you of?
How did the Sumerians navigate?
http://kahalyahweh.net/Articles/chirho.htm
“Though modern representations of the chi-rho sign represent the two lines crossing at ninety degree angles, the early signs of the labarum cross at an
angle that is more vividly respresentative of the chi formed by the solar ecliptic path and the celestial equator. This image is most familiar in Plato's Timaeus, where it is explained that the two bands which form the world soul (anima mundi) cross each other like the letter chi. Not only did the two legs of the chi remind early Christians of the Cross, "it reminded them of the mystery of the pre-existent Christ, the Logos Theou, the Word of God, who extended himself through all thing in order to establish peace and harmony in the universe," in Robert Grigg's words. Hugo Rahner summarized the significance:
"The two great circles of the heavens, the equator and the ecliptic, which, by intersecting each other form a sort of recumbent chi and about which the whole dome of the starry heavens swings in a wondrous rhythm, become for the Christian eye a heavenly cross. Of Plato's image in Timaeus, Justin Martyr, the Christian apologist writing in the second century, found a prefiguration of the Cross, and an early testimony may be the phrase in Didache, "sign of extension in heaven" (sēmeion ekpetaseōsen ouranō).
Not only was this pagan symbol well known in the pagan world centuries before our Savior even came into the scene, it was associated with something that is completely ignored…and for good reason! Note the “S” wrapping the bottom portion of the verticle pole in the above picture. This is the ancient astrological sign of ‘serpens’, the symbol of the serpent healers!”