There were three Persian kings named “Artaxerxes”: Artaxerxes I (464–424), Artaxerxes II (403–359), and Artaxerxes III (358–337). The king in this passage is Artaxerxes I.
The letter was probably dictated in Persian to a scribe, who translated it into Aramaic and wrote it down in Aramaic script (see Introduction: Text). Aramaic was written in an alphabet borrowed from the Phoenicians. In the eighth century B.C., this became a distinct script that remained quite uniform till after Alexander’s conquests