A few weeks back, a buddy of mine were talking about our teffillin. He picked his up in Israel, and my teffillin hails from from Crown Heights in Brooklyn. We both enjoy the mitzvah of teffillin and
enjoy talking about teffillin. You only buy one set for life. Anyway, he told me something I didn't know that the arm strap has a special knot shaped like a yud that fits into a special notch in the protective case for the arm bound box. I knew there was this notch, but what it was for was lost on me. Indeed, the knot fits perfectly.