According to John Loring, the B stamp seems to have been used only on "Absorbal" pipes from 1910 to (possibly) 1924, with the A costing six shillings six pence and the B costing eight shillings and six pence. He mentions that very limited straight grains might have been stamped B but also admits that he'd never seen one and thus judges it more likely that they were stamped only with the well-known DR stamping. For Dunhill's Bruyere pipes during this first period (c. 1910-1920), however, the A stamp indicated the first quality. In 1918, though, Dunhill began to add various stops after the A, sometimes a period, sometimes a small circle, sometimes a small rectangle or two. Until 1923, Dunhill also sometimes circled the A. John Loring speculates that "a circled A was the base grade with an uncircled A and an uncircled A followed by a round stop being the next two steps up" (pp. 37-38).
Sorry to post late, but I had to register an account. Happy to join the forum, though. I can post a photo of another encircled A Bruyere from 1922 if anyone is still interested.