Apple has just seeded a new build (13D55) of OS X Mavericks to developers. The update is currently showing for users on the beta from the Dev Center and doesn’t seem to be showing up yet for members of the recently-reopened AppleSeed program. This release is expected to include improved USB syncing in iTunes and enhanced support for 4k monitors on newer MacBook Pros.
Typically Apple increases the frequency of beta builds as it gets closer to the public release of a piece of software. The previous OS X build was released one week ago, indicating that Apple is possibly preparing to release 10.9.3 in the very near future. The full release notes for this build are below, and once again point developers to areas such as graphics and audio drivers, Safari, Mail, and iTunes USB syncing of contacts and calendars as key areas to test.