Concerning Samekh
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I believe it is a reference to the Temple design in Book 4.
Within the Magick Circle he provides as an example is inscribed a Tree of Life
This Tree of Life has each Sephira represented by a Square
With an overall shape of the Lingam
Inside each square is the astrological sign associated with the Sephira it is meant to representSo...
In the "Square of TIphereth" is the sign of the Sun, a Circle.
That is the "Circle within the Square of Tiphereth" inside the Magick Circle. -
By "Samekh," I assume you mean Liber Samekh, yes?
He's referring to the standard A.'.A.'. initiation temple floor plan given in Book 4, Part II, Chapter 2, "The Circle."