Cyrillic fert writes the same consonant and takes the same numeral, five hundred, inherited from Greek phi where Glagolitic counts to it. The pair is one of the letters where the two alphabets agree on the number, the sound and the position and where the name, in both scripts, is a word nobody can explain.
That is not a small point. The names of the first twenty letters are ordinary Slavonic words, and their sense is what makes the recitation memorable and what supports every reading of the recitation as a connected text. From this letter to the end of the alphabet the names stop meaning anything recoverable. The reference works list them without glosses, and no systematic etymology of the whole set has been carried through.
Two explanations are available and neither has been demonstrated. The names may be Slavonic words corrupted past recognition by centuries of recitation, or they may never have carried a sense at all, having been coined or adapted as labels. A third possibility, that they reflect Greek letter names filtered through Slavonic mouths, has been raised for some of them. The atlas records the absence as an open question, since an absence of this kind is worth stating rather than papering over.