Daf Yomi - Sukkah 25 - Pe'er-Tefillin shel Rosh only?

Rav Dovid Goldberg quotes Rabbeinu Avraham min HaHar who writes that pe’er of Tefillin is because it is said vrau kol amei haaretz ki sheim HaShem nikra olecho veyaru mimeka, and the Gemara in Brachos states these are Tefillin shel Rosh. Rav Goldberg wonders then why an aveil is exempt from Tefillin shel yad which does not seem to be included in pe'er. Rav Goldberg suggests that the passuk that states veyaru mimeka refers to Tefillin shel Rosh because they are visible, whereas the Tefillin shel yad are not visible. Rav Goldberg concludes that in reality, even Tefillin shel yad are referred to as pe’er. The Meshech Chochmah at the end of Parashas Bo writes that we say in Vehu Rachum on Monday and Thursday ad masai uzcho bashvi vsiferatecho beyad tzar. Oz is Tefillin shel yad, and tiferes is Tefillin shel Rosh. The Meshech Chochmah seems to imply that tiferes-pe’er only refers to the Tefillin shel Rosh. Yet, the Meshech Chochmah himself in parshas Beshalach writes that both oz and pe’er refer to Tefillin. Perhaps when oz and tiferes are written together, one can distinguish between the Tefillin shel yad and the Tefillin shel Rosh. Rashi in Ta’anis 16a and other Rishonim in Mode Katan, Kesubos and Bava Basra seem to imply that pe’er refers only to Tefillin shel Rosh. I also wonder if there is a distinction between tiferes and pe’er, which seem to be the same shoresh, yet imply different connotations. The Mishna Berurah in Hilchos Tisha B’Av quotes the Medrash that states that the passuk that states hishlich mishamayim tiferes Yisroel refers to Tefillin, and that is why we do not wear Tefillin on Tisha B’Av morning.