Any one of the many stabilised Buckfast lines mated with another of these Buckfast lines. II or isolated mating site needed to be sure you have the cross you want. Really B+ ....as a breeder of queens I'd have thought you would have known that
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Be warned a lot of queens are sold as Buckfast that have no trace-ability to any of the current lines listed in the Buckfast breeders directory. And even some of the genuine sellers are of the stack 'em high sell 'em cheap with little quality control. That also applies to the Carniolan and Italian strains they also sell.
Caveat emptor....