The halakhah requires that two shin`s are manually embossed, on two sides of the head Tefillin:
A three-headed letter shin is embossed in the side of the ketzitzah to the right of the person putting on the Tefillin.
A four-headed shin is embossed in the opposite side (to the left of the person putting on the Tefillin) of the ketzitzah. If one looks closely, it is possible to see an additional three-headed shin, formed of the spaces between the four heads.
The difference between the two shin letters has roots in Jewish tradition that are both profound and sublime:
The four arms of one shin and the three arms of the other together total the number seven, which is highly significant in Judaism: the seven days of Creation, the seven days of the Festival of Passover and the Festival of Sukkot (in Eretz Israel), the seven weeks of sefirat ha-Omer (the counting of the Omer), the seven years of the Sabbatical year cycle, and the seven times seven-year Sabbatical year cycle (= 49 years) between Jubilee years. The numerical value (gematriyah) of the two shin`s, by the following calculation, totals the number 613, which is also the number of commandments in the Torah.