The Old Alphabet is found and used in Telepath RPG: Servants of God. You learn how to decipher it in Crypt 3, with 2 rooms that, after scanning it, the player has both the translated and non-translated versions. Then using what letters you know and the other non-translated messages you find in the crypt, you can start to fill in the blanks.
While to complete crypt 3 you need to use the Old Alphabet, it can also be used to open certain chests.
X in the alphabet looks similar to a 3 with the lower section much bigger that in the upper
When you first come across the Old Alphabet, Duvalier remembers back to when he used to complain about having to learn it (which he never did) as no one uses it anymore.