Stephen A. Kaufman offers a transcription of Targum Pseudo-Jonathan for Genesis 4:15 that discusses Cain's "mark" or "sign."
Stephen A. Kaufman, ed., Targum Pseudo-Jonathan to the Pentateuch. The Jewish Literary Aramaic version of the Pentateuch from the files of the Comprehensive Aramaic Lexicon Project (CAL) (Hebrew Union College-Jewish Institute of Religion, Cincinnati, USA, 2005), Genesis 4:15. Electronic edition, accessed December 21, 2022
ורשם ייי על אפי דקין אתא מן שמא רבא ויקירא בגין דלא למיקטול יתיה כל דישכחוניה באיסתכלותיה ביה
English Translation (Maher): Then the Lord traced on Cain's face a letter of the great and glorious Name, so that anyone who would find him, upon seeing it on him, would not kill him.