Leon,
You're a D303 collector for goodness sake. If Mother's and G.B. backs share the same designation, then so should T213-1 with T206. D303 G.B.s are "yankee" cards and the Mother's are "rebel" cards.
As Leon mentioned we've had a lot of good discussion on the T213-1 in the past. I was one of the bandleaders saying it should be part of the T206 "set."
The fact of the matter is that T206 is not a set.
If you want to argue about Burdick's designations then yeah maybe he should've gone with:
T206-1 (piedmont)
T206-2 (sweet cap)
T206-14 (coupon) etc
But, I think there's a great chance Burdick was very familiar with how similar type 1 Coupon's were to other "T206" backs as far as ornamentation and font color on the front. Those are pretty basic traits. He probably decided it would be easier for collectors to associate the type 1s with other coupon backed issues.
However, if we have to group T213-1 and T215-1 with T206 so that our collections of those type 1 cards are now worth more, then I agree that there's enough evidence to do so.
One of the "other sides" arguments had been:
Argument for Paper stock. It differs from that of all other brands.
Counter argument. American Beauty size differs from that of all other brands.
There are more arguments against that one can retrieve by looking through the old threads, but none hold up in my opinion.
Rob