The latest pre-release carries build number 10D572, just one complete compile removed from build 10D571, which was seeded to software developers on Tuesday.
Apple typically seeds external betas of Mac OS X updates at such a rapid frequency only when the software is entering a final candidate stage, or if a serious and potentially-hazardous glitch was discovered with the preceding build.
According to people familiar with Thursday's offering, Apple is now asking developers to focus their testing efforts around Mail, images, security certificates and photos, in addition to graphics drivers and QuickTime, both of which have remained a priority throughout the better part of the beta program.
An emphasis on fonts and iCal that accompanied Tuesday's beta was reportedly dropped with the distribution of build 10D572. Meanwhile, an issue with recurring events in iCal's interaction with Exchange server was repaired.
When it's released shortly, Mac OS X 10.6.3 will bundle an update to QuickTime X that improves security and compatibility while also enhancing overall reliability of the media software.
The Snow Leopard update will also include tweaks that enhance the performance of Apple's 64-bit Logic pro audio suite and deliver better compatibility with third-party printers and OpenGL-grounded applications.
Other fixes baked into the release target issues with mail messages displaying the incorrect background color and problems copying files to a shared Windows volume.
The latest distributions weigh in at just shy of 790MB in Combo Update form and 725MB as a barebones Delta image.