When you order online by system information, you are shown what we refer to as an “external” part number, in your case number CT518508. It is a system specific number and is mapped exclusively to the Dell Latitude D620. The longer part number, CT25664AC667, is the one that you see printed on your module, and that is the actual module part number, or the "base" part number. The two numbers are in other words two different ways of referring to the same part.
Please note that we can only guarantee compatibility if you order by system, since we sometimes need to filter out a certain build of a part. If you order by specifications we don't know what model of computer you have, and that makes it impossible to guarantee compatibility.
