Final Tzaddi (tsadi sofit) is the word-final form of the simple letter Tzaddi, straightening into a long descender. In standard gematria it equals Tzaddi (90); in the extended mispar gadol reckoning it completes the series with the value 900. As a variant of Tzaddi it inherits that letter's sign (Aquarius, and the trump The Star in the original Golden Dawn overlay) rather than an independent correspondence.
The five final forms are a detail the Sefer Yetzirah itself is silent on: the text as it survives knows twenty-two letters, three mothers, seven doubles and twelve simples, and says nothing about separate word-final shapes, so Final Tzaddi's classification is inherited wholesale from Tzaddi rather than being the text's own assignment. That base text is itself hard to date: Gershom Scholem placed its composition somewhere between the second and sixth century CE, and it survives in three recensions that disagree with each other in detail.
The Hebrew script carrying these letterforms is not the script of the earliest Hebrew inscriptions either; it develops the Aramaic chancery hand of the Achaemenid Persian empire, adopted after the Babylonian exile and standard by the Hasmonean period, displacing the older palaeo-Hebrew script, which survives afterward mainly on coins. The letter names are Northwest Semitic words inherited from that older script; their familiar pictographic glosses are modern reconstructions, and the names travelled into Greek unchanged in sound but empty of meaning.