This question explains that the extra hair emanating from the tefillin shel rosh of Rabbeinu Tam tefillin is a way to identify the shel rosh of Rabbeinu Tam tefillin vs. Rash'i tefillin.
If 2 tefillin shel yad were next to each other, one Rash"i and the other Rabbeinu Tam, how could one distinguish them from their exteriors, if at all?