“I haven’t a clue. . .”
You can check this by clicking “About this Mac” under the Apple menu.
In the window that opens, click “more info...”
Towards the bottom, it will say “Software OS X 10.8.3 (xxxx).
The xxxx is the build number.
Oh, I know, but I haven’t been home to update my Mac, so, no way to do that. Sorry.