@ytc1 @SolarisDiaspora the DTrace source code was released at the same time, as @bcantrill described in his blog back then: https://bcantrill.dtrace.org/2005/01/25/solaris-10-revealed/ - while the rest of the ON source came later in June of that year.
And yeah, January 31 was when Solaris 10 FCS went up for download, though the press release didn't go out until the next day: https://web.archive.org/web/20050212152357/http://www.sun.com/smi/Press/sunflash/2005-02/sunflash.20050201.6.html