I would think that attacks from the rear would be most easily defended since you have no deflection and the closing speed is lower. With the ballistics of the .50BMG you could reach out far. But the flip side is that the attacker has the same advantage with a more stable target.
Now question for Civvetone... are those bomber box formations applicable to all USAAF makes? Were defensive formations different between B-17s and B-24s? Certainly, these different airplanes had different operational altitude capabilities. Does operational altitude change defensive formations?
I'm wondering how to explain the signficantly higher percentage of attacks upon a particular flank depending upon airplane model. At first I was thinking it might be Luft taking advantage of engine torque for disengagement techniques, but that would only make sense if they were consistent between models.