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http://www.darkfiber.com/tomb/htwsstks.html

the letters on the capstone on the gravemarker
should be read clockwise, starting at 12 o'clock:
H T W S S T K S.

(many of us non-Masons start at 6 o'clock
  and get a different combination:
  S T K S H T W S.)

        "the candidate carries represents a keystone, and has the
        initials H. T. W. S. S. T. K. S. engraved upon it in a circle. ...

            Here the Right Worshipful Master calls the candidate's attention to the keystone before him, by pointing out to him the initials on the stone, which he is informed read as follows:--
                HIRAM, TYRIAN, WIDOW'S SON,
                SENDETH TO KING SOLOMON."

	h.t.w.s.s.t.k.s. or s.t.k.s.h.t.w.s.

Solomon, king of Israel
Hiram, king of Tyre (city in Lebanon)
Hiram Abiff, our Grand Master, perhaps the greatest Mason the world ever knew,
                        who was slain just before the completion of King Solomon's Temple.

Jubela, Jubelo, and Jubelum, brothers from Tyre, killers of Hiram Abiff.