"According to my sources at Microsoft, the software giant has set its sights on July 27, 2005 as the release date for Longhorn Beta 1, though obviously that date could slip if the company is unable to hit internal bug requirements. The current escrow build for Longhorn Beta 1 is 5101, a few builds older than build 5103, the current internal build in the Beta 1 fork.
As reported previously, Microsoft recently forked the Longhorn build process to segregate Beta 1 code check-ins from the post-Beta 1 (or what we can think of as the Beta 2) code path. Recent leaks of Longhorn builds are from the Beta 2 code path, but we can expect them to be quickly eclipsed by the Beta 1 code base as Microsoft makes that milestone widely available."
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Very interesting stuff, but my understanding is Longhorn Build 5203 is a post BETA build that could eventually be used as an interim update to the initial BETA 1 or the PDC Developer preview.