Empirical fitting models for the Pop. B 1900 Å blend. Upper left: specfit model (M3) of the composite spectra of Pop. B sources with VBC (in red) added to the profiles of Al III, Si III], and C III]. Abscissa scales are rest frame wavelength in Å. The ordinate scale is the specific flux in units of 10⁻¹⁷ergs s⁻¹ cm⁻² Å⁻¹. Upper right: synthetic model (M4) with BC+VBC in all three lines with the median value (dashed line) and the uncertainty regions of each profile in light and dark tones: red, green, and blue for Al III, Si III], and C III] BC – VBC, respectively. Grey lines are the M3 flux values as the initial condition. Lower panels: Gaussian distributions of the BC - VBC fluxes obtained with a million random iterations of M3, with values that satisfy the condition of χ²(M4)/χ²_min(M4) < F(2σ).