The two shins — one written, one engraved

A shin is pressed into the leather on both sides of the head-box, raised from the hide itself rather than painted on. And the two are not the same letter.

On the wearer's right: an ordinary shin, with three heads, the letter as it is written everywhere else.
On the wearer's left: a shin with FOUR heads, which exists nowhere in Hebrew writing except here.

Both are required. A box with two three-headed shins is wrong.

⭐ The reason the scribes give is the best thing in this room, and it reaches straight into the room next door. The three-headed shin is the letter as written — ink laid onto a surface, material added. The four-headed shin is the letter as it stood on the tablets at Sinai, and the tablets were not written, they were cut — the letters made by taking stone away. Two shins on two faces of one box, because there are exactly two ways a letter can come into existence, and this object carries both.

⭐ Which is the identical problem the Shamir solves one room over: iron was forbidden on the Temple stones, so the letters of the breastplate had to be opened by a mouth — a cut that removes nothing. Three doors, one spine. Also offered, and older in the popular literature: three heads for the three patriarchs and four for the four matriarchs; and the reading this app already carried before this room existed, in Shin's own entry — a fourth flame held in reserve for the world to come. Those are readings. The written-and-engraved account is the sofrim's working one.