in the datebase the set is 50 cards and since this is #51 I imagine it's an extra or promo type card. They are listed in regular, blue, green and red so I am assuming it's just not in the database. (normal for Beckett to be missing cards) From what is listed in the database it would seem there a A, B and NNO promo versions. It would be listed like this if it was though.
2008 Donruss Threads Pro Gridiron Kings Framed Black #51A Brett Favre dropping back