I was just reading some GI documentation on the mustang's fuel system last week, it refered to the 75 gal and one size up (I think 85 or 95) combat drop tanks and made a seperate reference to larger paper ferrying tanks.... I was busy with the task at hand but just made a mental note of it. I haven't had any time to follow through since unfortunatley.
Also I stumbled across an original wiring spec for the electrical system that solely dealt with the rocket ignition system, when they upgraded to the HVARs from the tubes (6 rocket capacity) the new launch system was a 10 rocket capacity, standard (5 each wing, odds one side evens the other).
So Challenge may be on to something with his request, if our mustang is that early it should have the tube option, otherwise it's a later model and is missing 4-rockets.