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PCOn Scot Reader's analysis, which I think is correct, the Cobb/Cobb back is not part of the T206 set.
If you want to add it, then you need to add Coupon backs and Red Cross, etc., which are (properly) not categorized as T206s.
Why stop there? If the T206 set is whatever you want it be, then let's include Obacks. They are close enough.