שמות כח,יז: וּמִלֵּּאתָ בוֹ מִלֻּאַת אֶבֶן אַרְׁ בָעָה טוּרִ ים אָבֶן
The Gemara (Yoma 73b) states that each of the 12 stones (four rows of three stones) had 6 letters etched into it. Obviously, the names of the 12 tribes fall far short of containing 72 letters. The Gemara says that besides the names of the tribes, the names of our Patriarchs and the words "shivtei Yeshurun" were also etched into the stones. The Baal HaTurim explains that the names of the tribes (total of 50 letters) were first etched into their respective stones and the amount of letters short of 6, anywhere from 1 to 4, were completed by adding 13 letters from the Avos (אברהם יצחק יעקב) and adding the 9 letters from "shivtei Yeshurun (שבטי ישורן)." The one exception was Binyomin, which contains six letters and needs no supplementing.
לוירהם שמעוןב ראובןא
זבולןח יששכרצ יהודהי
גדשבטי נפתליב דןקיעק
בנימין יוסףרן אשרישו
The name Yeshurun (from the word ישר – upright) is another name for Yisrael. It is not clear why this 5-letter name is used rather than the more common 5-letter name ישראל.
Perhaps this can be explained in a homiletic manner. The "choshen mishpat" contained the names of all 12 tribes on one breastplate. This represents unity of the tribes. Our Patriarchs empowered our nation with unity. The addition of the words "shivtei Yeshurun" further stresses the concept of their all being unified in their service of HaShem. This is stressed because the sons of Yaakov hated Yosef in varying degrees and sold him as a slave. Binyomin had absolutely no part in throwing Yosef into the pit, nor in selling him. He therefore needed no addition of the Patriarchs letters, nor letters from the words "shivtei Yeshurun" to his name. Possibly, a mathematical allusion can be added. The numerical value of "sinas Yosef," hatred of Yosef, equals that of "shivtei Yeshurun" plus the 13 letters of the names of our Patriarchs.
The Urim and Tumim were used to convey HaShem’s answers. Thus, they needed all the letters of the Aleph Beis. The letters of the names of the 12 tribes did not contain the letters ח ט צ ק. These letters were contained in the names of the Avos and of “shivtei Yehurun.”
