Where to find the build number INSIDE an install DVD of Mac OS X

It's not written on the DVD itself and I don't have the DVD sleeve anymore.

Not exactly through the DVD as you asked, but Apple provides a list will all the build numbers for the Retail installation disks.

Once you've run the Installer, you could always go to the Apple Menu () and choose About this Installer, which should provide the build number.

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.