According to Joystiq and Kotaku, MS started sending out emails to your xbox live linked email accounts with Codes to redeem for the Specializations. You only qualify if you A) Played Halo 4 before November 20th and B) played it while connected to Xbox Live.
Although I followed the steps mentioned above, I for one, haven't received it but if you got it, please reply. If anybody knows how to get the codes by contacting MS or a certain xbox channel, please notify below!
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If you haven't received your code, you have to go to the following link and enter your information. I hope this helps all who haven't received their codes yet.
Post the following:
*Confirmation (by saying yes or no) you played the game while connected to Xbox LIVE before 11:59:59 P.M. Eastern Time on November 20, 2012
*Confirmation (by saying yes or no) you are 17 years of age and older
*Confirmation (by saying yes or no) you live in the United States, Canada, Chile, Mexico, Colombia, Brazil, Japan, Hong Kong, Singapore, Korea, Taiwan, Australia, New Zealand, or India