Every other reseller site I go to, lists a model number, such as "MGX82LL/A" but there is no model number on Apple.com. Given this model number, MGX82LL/A, how do I determine if this is latest or year old or two year old? any clues? Also I see two different model numbers on same macbook pro - MGX82LL/A vs A1502. What are these and how are they different?