Where to find the build number INSIDE an install DVD of Mac OS X
Check the SystemVersion.plist file that is on the Install Media.

Within the install media is the file containing the Build number, it is located at /System/Library/CoreServices/SystemVersion.plist

The string value for the Key ProductBuildVersion is the information you are looking for.

Note: This tip is useful for getting the Build versions on any OS X bootable disk, regardless of install ability.