The final version is 6.5.1.f. Before final, we have 6.5.1.a (alpha). b (beta) and c (candidate).
Alpha become beta after all new features implementing in alpha. At this moment, ApacheTrafficServer (another cache server like Varnish). Dovecot (as Pop3/Imap4 alternative to courier-imap in qmail) and multiple-php not implenting yet.