So, the AAF thought the Brits had included too much armor, right? Now drop the nose armor to save another 100#.
How do you get that?
The Americans used a different oxygen system that didn't require bullet proofing. The Americans removed 62.61 lbs of armor from the the oxygen set up and yet the total armor weight difference was only 53.02 lbs. so the Americans, with a bit of juggling the other pieces, were using 9.6lb more than the British once the armor for the oxygen tanks is taken out.
As to the radios, the radios changed between the different types/models and between countries. The British may have started fitting IFF gear before other countries. One diagram of a P-39 Q shows radios both in the original position and above the the engine but doesn't say what kind of radios they are. IFF gear tends to get lumped in with radios instead of being called a radar transponder.
From AHT just to show how confusing things are.
Category,,,,,,,,,......................,,,...P-39D..........P-39D-2............P-400
Elec system.................................225..................274.6.................278
communications........................62.....................80.2..................35
Misc. Eqpt (radio)......................----................129.2...............109.9
what piece of equipment got thrown into which catagory changed over time. I am not sure about the lack of radio weight for the P-39D. radio was not counted in basic weight? sample aircraft did not have radio fitted? I have no idea what "communications" covered if the radio weights are listed elsewhere.
The roughly 50lb increase in electrical weight is interesting. It cover quite a few things.
Voltage requirements for different pieces of equipment varied and things like this were used to convert it.
What catagory this goes in (electrical or radio) I have no idea.
people have already said that the old radios could be tempermental.
Some of us are old enough to remember even car radios being affected by poor ignition wires on car engines. Shielding the radios from static from the ignition system or other electric devices in the plane took some doing. They got a lot better at it fairly quickly but again, just because they could do something in 1943 doesn't mean they could do it in 1941.
Sometimes "the" radio was in one box, often you had a radio tray or box that contained several separate pieces of equipment or chassis. Sometimes descriptions, especially if translated are not real clear on exactly what was done.
Before you get too crazy about removing the nose armor please remember that when they went from the P-39C to the D/P-400 they also yanked about 50lbs of .30 cal guns and about 60lbs worth of ammo out of the nose. Plus accessories.
Larry Bell and company did build one really stripped aircraft which I have already mentioned but there doesn't seem to be record of it's CG location. And it was flown by how many pilots? a few company test pilots?
What is acceptable flight behavior with an aft CG location varies from country to country. Or even in the same country depending on how severe the combat situation is.
Just because the Russians thought an airplane was OK to fly doesn't mean the UK or the US would.