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Apple has seeded the first beta of OS X 10.8.5, Build 12F9, to developers



From MacRumors:
Apple also released a developer preview of OS X Mavericks last week, which is expected to be released to the public this fall. The update is available through the software update tool in the Mac App Store and Apple's Developer Page.
  Apple Seeds First OS X 10.8.5 Beta to Developers