Luzim nuts hint to the concept of Leah. She is at the back of the neck, the first wife. Then afterwards one enters the Holy Temple, which is the concept of Yaakov called it Beit-El (the House of God) (Pesachim 88a).

This is as Rashi explains, that through luz one enters Beth-El (see Judges 1:24).

This is “Nachamu, nachamu (Console, oh console)” (Isaiah 40:1), [as] twice NaChaMU has the same numerical value as KaDKoD (fontanel)—which is the concept of the back of the neck through which one enters the House of God.

2. And this is the Three Weeks, namely the twenty-one days of Between the Straits. Among trees, the counterpart to this is the luz. This is the reason it is customary to eat an egg for the meal before the fast of Tisha b’Av. For an egg matures in twenty-one days, as our Sages, of blessed memory, taught: A hen lays her egg after twenty-one days. Among trees, the counterpart to this is the luz (Bekhorot 8a). This is also the luz bone at the back of a person’s neck. It will remain after the body decays, and from it the body will be built anew at the time of Resurrection of the Dead.

And that is our main consolation, the concept of “Nachamu, nachamu,” as mentioned above. For about the body it is stated “lie down until morning” (Ruth 3:13). This refers to the time of the Resurrection. “And when the morning came, look, she is Leah!” (Genesis 29:25)—i.e., the aforementioned luz bone through which the body is rebuilt.

3. And because Yaakov’s entire focus was Rachel, it is said of Rachel’s sons: of Yosef—“on the kadkod of one set apart from his brothers” (ibid. 49:26) ; and of Binyamin it is written—“and between his shoulders He dwells” (Deuteronomy 33:12). This is a clue that it is the concept of Leah, who is the first, which enables one to attain the concept of Rachel. “Kadkod” and “his shoulders” allude to the aforementioned luz .

4. The numerical value of ORePh (back of the neck) is Shin Nun (350), the first letters of Shabbat Nachamu; alluding to that which was mentioned above.