Cyrillic myslete writes the same consonant, holds the same place and takes the numeral forty, the value of Greek mu. Glagolitic gives it sixty. The gap of twenty between the two figures is the accumulated effect of every Slavic letter earlier in the order that Glagolitic numbered and Cyrillic did not.
The name is the same verb in both scripts with a slightly different vowel in the middle, myslite beside myslete, and neither form can be shown to be the original. The names travelled by recitation, and a recited list picks up small changes in a way a copied text does not.
For anyone comparing the two alphabets, this is a convenient letter at which to stop and see the pattern whole. In fifteen letters the two scripts have kept the same order, the same sounds and the same names, and have shared none of their shapes and almost none of their numbers. Any account of their relationship has to explain both halves of that. An account that makes one script a modification of the other has to explain why nothing of the appearance carried over. An account that makes them independent has to explain why the order and the names agree so exactly.
Neither half is easy, and the evidence that would settle it does not exist in the form anyone wants. There is no manuscript from the mission, no abecedary from the ninth century, and no statement in the early sources that names either script by a term a modern reader can be sure of.