The code B indicates the approximate inside diameter of the ring. All ring sizes starting with B have approximately the same inside diameter.
Most B rings are made by wrapping wire around a 3/32" mandrel (rod). The inside diameter is thus slightly more than 3/32 inch.
In order to keep sizes consistent across metal types, softer metals such as copper are often wrapped around a mandrel that is ever-so-slightly larger, and more resistant metals such as stainless steel may be wrapped around slightly smaller mandrels. This means that the final ring size for all B rings has very little variance from metal to metal. The Aspect Ratios are so close that all metals of this size can be used interchangeably.