Responsa do Kapłańska 10:3

They responded as follows: "These and these are the words of the living God!" (Eruvin 13b) Just as you dispute below, so we dispute above. For the Holy One, blessed be He, holds that the paragraphs beginning with the words 'vehayah' should be in the middle of the tefillin, and all of the heavenly host say that they follow their chronological order. "This is what He meant in saying, Through those near to Me I show Myself holy, and gain glory before all the people.” (Leviticus 10:3) - His glory is in the paragraph of his Kingship being first.^*This was a widespread debate throughout the Jewish world. See, for example, a question sent from the community of Lunel to Maimonides asking a similar question to the one posed here, quoted in Kessef Mishneh on Mishneh Torah, Tefillin 3:5. The last comment seems to allude to the opinion of Rabbeinu Tam, with the paragraph of the Shema (termed a phrase of Kingship in Rosh Hashanah 32b:16) on the outermost side, the first one from the perspective of the one wearing the tefillin on their head.