I am convinced, purely from tasting (which I realize is not empirical evidence), that Brandy Boy is derived from Brandywine, but is not Brandywine in a new wrapper.
The fact that the two plants look so similar (Brandy Boy leaves have a bit more serration, the larger leaves look like oven mitts vs. the Brandywine leaves which are completely rounded), and have the same growth habits.
Brandy Boy has an excellent flavor, but not identical to Brandywine.
The fact that Mischka and Suze have both grown F1 and F5 side-by-side and found them to be identical tells me that either Brandy Boy is NOT a hybrid and they are just pulling our leg (and our wallet to the tune of $2.50 for 10-15 seeds), or it is a hybrid with so little variation in the F2, that there is really no reason to buy seeds of it again.
So now I am down to being dependent on the seed companies for just Jet Star and Sungold.
By the way, Brandywine and Brandy Boy which are 3 plants apart in my garden, each have 10 fruit including ones I've picked. The Brandywine fruits are monstrous in comparison. Brandy Boy has yet to distinguish itself from Brandywine for me as far as noticeably increased productivity.