AppleInsider reader Tom noticed a Webkit bug report submitted through Apple's Radar system that references an operating system build number of 13A451. The first three characters of that number are believed to refer to OS X 10.9 and the last three characters are the internal build numbers.
Interestingly, the last three digits are surprisingly high for an operating system that has not yet been released. The final release of OS X 10.8 Mountain Lion was released after only 269 builds. OS X 10.6 Snow Leopard was released after 432 builds.
Major OS X releases usually have higher build numbers. For example, OS X 10.5 reached build 581 and OS X 10.7 Lion reached build 511.
This could suggest that either Apple is nearing a release for the operating system or that a critical component is seeing a major update.
Last year, Apple released a developer preview of its upcoming operating system in February; however, it did not do so this year. It's possible that the company plans to unveil a nearly finished version of the software at WWDC and have a much smaller test window before its public release.
OS X 10.9 is reportedly codenamed 'Cabernet' and is said to introduce new 'power-user' enhancements and borrow core iOS features. Major enhancements to Finder such as tags and tabbed browsing modes, a new Safari web browser, and the ability to keep different Spaces open on different monitors, is expected.