Some time in the 1990s Buffet started stamping the model and key on their barrels and bells that shipped with their clarinets.
(I.e. you have a Bb R13, a Festival would mention that, an RC Eb would say RC E, so on.)
Yes, like TrueTone says, the "B" indicates that it's a bell for a Bb clarinet.
Which is odd, because, despite some accounts to the contrary around the internet, I'm not sure that they are actually different. I just put the bells from my Bb and A clarinets (stamped "B" and "A", respectively) onto a level surface and a level placed across the two reads perfectly flat. They both fit on both instruments and the diameter at the end of the flare is exactly the same. I don't have sophisticated enough equipment to measure the inside diameter, so I can't speak to that.
From the outside, the shapes don't look different to me. I'll have to try swapping them next time I play and seeing if there is any difference at all.
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