Inferred M_CO (within uncertainty bounds) of the most luminous Type IIb and Type Ib SN progenitors. To obtain the estimates, we use the parametric scaling law log L_TACCB(M_CO), which remains reliably applicable – at any Z – up to log L_TACCB/L_⊙ < 5.2. For higher SN progenitor luminosities, the scaling law provides a lower limit on M_CO of the SN progenitor. Observations are compared with the direct BH formation windows for Case B donors predicted by our CCSN recipe.