Is there trully a “Low” B flat as well as a high B flat or are both of these the same whistle in the same octave?
Conventionally, Bb without further qualification would refer to the whistle keyed a Major 3rd below the standard D whistle - i.e. the Gen Bb and its peers. Low Bb would be 1 octave below that, but fairly uncommon because of the size.
As a rule of thumb, take the Generation range as defining the “high” whistles, and anything below that is “low”. Only the A whistle is ambiguous, so “alto” or “bass” can be used to distinguish the two realistic possibilities.