Are you saying the additional weight came from structural strengthening sometime after production of the C model in time for the D model and P-400? According to your P-39C manual empty weight was 5016lbs. The P-400 weighed 5550lbs empty. That's a difference of 534lbs. 240lbs was the self sealing fuel tanks. The book "Cobra" shows "minor empty and useful load changes" in Jan '41 and "engine and fixed equipment weight increase" in June '41 with production beginning in July, but those changes only account for 165lbs. I don't know where the additional 129lbs came from. Maybe it WAS structural bracing.
I
think some of the weight was from bracing and/or thicker components.
The weight difference is greater than it seems. the P-39C empty weight included the radio, the empty weight of the P-39D-1 did not.
I found how much the radio in the P-39C weighed. 43.3 lbs
It was a different radio than used in the P-39D-1 or other US P-39s. This is the communications radio and not the IFF gear. The IFF gear was included in the 129lbs listed for the P-39D-1 but one website says the IFF gear weighed 45lbs.
The communications radio weighed 91lbs. Don't know if anything was shared.
Weights are for a complete radio set including the switch boxes in the cockpit.
The radio in the P-39D-1 was much longer ranged than the one in the P-39C, range is give as 130 miles at 10,000ft.
The one in the P-39C could receive ground transmissions at 30-45 miles, the ground station could hear the aircraft at 10-20 miles.
Some of the weight
might have been in stronger landing gear and/or different tires. The P-39C went between 7100 and 7300lbs full load (nothing external) compared to the 8368lbs of a P-39D-1 with 75 gallon drop tank. The load factor was not at max gross weight but at a lower "combat weight", this is my term. Like P-36s were rated with 105 gallons of fuel in the tanks (tank behind pilot not filled). But the landing gear/tires still had to handle the heavier loads.
There are a few things that need changing as the weight of the plane goes up.