The following is most likely false, since I do not really know enough about the field to know if this would be correct, or what I am proposing would be wrong, but I've been thinking about the following recently - the origin of the Greek letter zeta. Everyone equates it with zayin, but I would like to consider the possibility that it comes from tzadi.
Firstly, here is the wikipedia article on the subject of Zeta.
Why should we think it is == to the English letter "z"? Well, in modern Greek, it is pronounced "z" (= the "z" in the Internation Phonetic Alphabet). Furthermore, it has the value of 7 in Greek, being the 7th letter (the Greek letter digamma, = "w," preceding it). Thus, we have:
digamma (actual name unknown, since it fell into disuse, probably called waw) (=vav)
etcetera (and no, etcetera is not a Greek letter)
Thus, the Greek letters parallel the Hebrew letters. Epsilon is an exception. The argument can be made that eta and theta are simply chet and tet. But, zeta follows this pattern, and the latter is not called zayn.
Yet, the position of the letter, together with its current pronunciation, makes it quite likely that the letter = zayin. Wikipedia states that it derived from Phoenician zayin, but took its name froim the eta-theta pattern. And the Roman z and Cyrillic З derive from it.
Why would I put forth צ, tsadi as a possibility? Several reasons.
Firstly, the name of the letter. It does not match the Phoenician (and Hebrew) name zayin. It does, however, quite closely match tsadi, ṣādē.
Secondly, we see in German that the the letter zeta, written "z," stands for the tz sound. To get our English "z" sound, German uses "s." Indeed, in Yiddish, when written in Hebrew letters, the letter tzadi is used to equal the letter zeta (z, pronounced ts) in Roman characters. Furthermore, in German, the letter is no longer in the seventh position, but is at the end of the alphabet. (Though perhaps one of the earlier letters assumes some other quality of zeta...) In Italian, zeta equals either a ts or a dz sound.
According to Wikipedia, is was probably pronounced [z] in Greek even in the Hellenistic age. However, earlier than that, it was not pronounced [z]. Rather, there is a dispute.
Some say it was pronounced [zd] as in Mazda, and some say it was pronounced [dz]. See the arguments for each over there.
Now, [dz] is simply the voiced equivalent of [ts], since [d] is a voiced [t] and [z] is a voiced [s]. And, we see that both these options are available in Italian.
Finally, if we look at the Greek alphabet, we see that where Hebrew and Phoenician have tzadi, Greek has nothing.
Thus, what might have happened - since Greek did not have a [z] sound by itself, it still wanted to match as much as possible the Phoenician alphabet, at least at the beginning. Thus, they moved tzadi and called it zeta, since it matched the pattern of the other two letters which follow, and gave it its usual pronunciation, perhaps though with a voiced equivalent. Thus, at the end of the Greek alphabet, a tzadi equivalent is omitted.