Baruch Shekivanti!

"In the beit midrash, it is considered an exciting delight to find that one's original thought was actually innovated by an earlier source, unbeknownst to the current thinker. This is usually taken to be a vindication of the thought patterns of the learner, and an exoneration of his logic. The happy student may exclaim, 'baruch shekivanti!', which ostensibly means 'blessed is He who directed me [to the same conclusion as source x]'. This phrase has become of modern parlance in orthodox circles, and is used in situations removed from torah learning, as well. (...)" Source

Exactly one year after my novel explanation about the Ot of Cain, which I identified to be the Tav just like the Ot of Mitzraim in Exodus 12:13 , I was shown a source to this claim - the Siftei Chachamim (see here, last line): אות ת רמז בו תחיה כלומר שלא יהרגנו

So it turns out that it was the letter Tav indeed and the Tav is in fact the short version of תחיה, "You shall live". My only source for that until now was Rabbi Kasher's chidush on the Odd letters in the Mezuzot, where he used the same logic. Now the Siftei Chachamim validates Rabbi Kasher's insight and confirms that the Ot of Cain is indeed connected to the Ot of Mitztraim. Fascinating.

It's just remains unclear if Hashem inscribed in Cain the letter Tav in Ktav Ivri or Ashurit. The Ot of Mitzraim was in Ktav Ivri, like an X. So did Cain have our traditional Tav or an X in his forehead?

Just an addendum, the Siftei Chachamim also brings that the letter could have been the Hey, one of the letters in G-d's name. As I mentioned in the original post, the Peirush Hasulam says that it was a Vav. I thought that was all - Tav, Hey and Vav. Then I looked in older Mikraot Gedolot and found this alternative version of the Siftei Chachamim (see here):  אות י רמז בו יחיה כלומר שלא יהרגנו. Somebody obviously messed up in the copying because that is NOT what the Siftei Chachamim says, so in case you see it, it's a clear Taut Sofer, copier's error.