B chamber is slightly smaller than the original chamber.
You can shoot the new brass in either chamber, but you will get some case expansion in the original chamber.
I don't think the original brass can be shot in a B chamber. And I definitely would not shoot fired brass from the original chamber in a new B chamber.
I think most gunsmiths that had an original reamer sent them in for a regrind to the B chamber specs. I would think the only way to get an original reamer is if it is owned by an individual. Even then, Inwould be reluctant to use it, as the new brass is undersized to fit the B Chamber now.