The B♭1 fundamental arises implicitly from F3 and B♭2 harmonics

Evidence

The f0 components of these two notes, that is, F3 and B♭2, form the second and third harmonics of B♭1, which would have been the next sounding note of the FIGURE 5. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1646