יג ויאמר קין אל יהוה גדול עוני מנשא יד הן גרשת אתי היום מעל פני האדמה ומפניך אסתר והייתי נע ונד בארץ והיה כל מצאי יהרגני טו ויאמר לו יהוה לכן כל הרג קין שבעתים יקם וישם יהוה לקין אות לבלתי הכות אתו כל מצאו
The Midrashim speak about this Ot / אות and there a few possibilities mentioned:
- Hashem gave Cain the gift of Shabbos, also called an "Ot" - [אות היא לעולם [שמות לא: יז., and by keeping the Shabbat Cain was guaranteed to survive.
- Hashem gave Cain a dog to protect him from the other creatures.
- Hashem marked his forehead with one of the Hebrew Alphabet's 22 letters (Rashi)
So it seems to me that there's room for interpretation here and I have an alternative option, based on the Torah Shelema's take on an old Minhag on Hilchot Mezuza. Please bare with me as I lay the background story before I move forward.
In the times of the Geonim there was a Minhag of writing additional mystical names in the Mezuza alongside the Mezuza's Parshiot. The ספר יראים brings it down here and you can see this odd Mezuza below:
The Rambam and others strongly opposed this practice and today this Minhag fell in disuse. But if you look carefully, you will see five letters written in an odd font. These are Ktav Ivri letters and the Torah Shelema asks how these Ktav Ivri letters found their way in the Mezuza, always written in Ktav Ashuri.
The answer is unique and extremely interesting. In the last Makah of Egypt, the angels came down to kill the Egyptian firstborns. Hashem told Am Israel to mark their door so the angels would not enter their door (exodus 12:13):
והיה הדם לכם לאות, על הבתים אשר אתם שם, וראיתי את-הדם, ופסחתי עלכם; ולא-יהיה בכם נגף למשחית, בהכתי בארץ מצריםThe simple reading is that the blood will be a sign, but here again some commentators say that the sign was a letter. More precisely, an X which is the Ktav Ivri letter for Tav, which symbolizes life (תחיה) [The Jews at that time only used Ktav Ivri and that's why they marked it like an X and not Tav].
Rabbi Kasher says that because the X protected the Jews in Egypt many started to add it to their actual Mezuzot for an "enhanced protection" - after all, the Mitzva of Mezuza is a remembrance to the Mezuza of Mitzraim, which was simply an X.
The five Ktav Ivri letters in the Mezuza are equivalent to תחאחא in our script and they mean תחיה אמן חיים אמן - a prayer for life. But the point is, that the X is the letter identified by the name "Ot" in the Torah.
If so, perhaps the unidentified "Ot" given to Cain is the same unidentified "Ot" that is mentioned by Yetziat Mitzraim - the X, or Tav. But why would Hashem give out of all letters the Tav?
The explanation is the same as in Yetziat Mitzraim: Tav is the initial of תחיה, "you shall live", and it was Hashem's guarantee to Cain that he would not be murdered - that he would live. I don't give a source to this connection between the Mark of Cain and Mezuza because it's my own Pshat!