If, in the letters, one sees the digamma, which looks like an F (because it's the origin of the Latin letter F). It wasn't used in writing but it was used, solely and exclusively, to represent the number 6. Similarly, qoppa (which looked like a Q) represents only 90, and sanpi (which looked just weird) represented 900. If you add those three letters to the 24 letters of the Greek alphabet, you get 27, or 3 sets of 9s, which were used for 1-9, 10-90 and 100-900, respectively.

On a 20-sided die, if the letters represent numbers, you wouldn't get to qoppa or sanpi, but you would get digamma.