Indeed MPN. I wonder what the crews were briefed to say in the event they were shot down and all 8 were captured. Why would there be 2 WOP/AG on board? I believe some of the Specials were of German descent and perhaps also Jewish? 2 pilots were explained easily by a new skipper flying a second dickie trip or two, but two WOPs would be in for a difficult time with interrogators.
It'd be interesting to know the effect on the CofG and handling of all that equipment and another body in the rear fuselage, or what was traded off - the only thing I can think of would be the ammunition for the turrets, as there's not really any space up front to add anything to counter.