Comparison between the number of evolved massive binaries resulting from fiducial models and assumptions of idealized mass transfer stability, in which all Case B binaries undergo stable mass transfer and produce an OB+cc binary. The left panel shows results for OB+BH (solid lines) and OB+WD binaries (dotted lines), with gray lines indicating fiducial results and blue lines indicating results based on the idealized mass transfer stability assumption. This panel is bifurcated with the major lower section retaining the same y-axis range as in Figure 3 to facilitate a direct comparison. The right panel demonstrates the results for OB+NS binaries with (dotted lines) and without (solid lines) SN kicks, again with gray lines corresponding to fiducial results and blue lines corresponding to results based on idealized mass transfer stability.