I was actually wondering about "the letter of Cefer" being Deggial's mark.
What exactly are the "letters of Cefer"? I could think of a couple of plausible ideas and they all involve "Cefer" being a mis-pronunciation. In Arabic, there are a pair of words that I can think of that correspond to a somewhat similar transliteration into Latin letters; one is "Sifr", the Arabic word for the number zero, which is the root for our word cipher. Another, more plausible, is the word "Kafir", which literally means "unbeliever" or "heretic".
So, what do you guys think?