What if the entire conversation Wayne Green recalls was about Bob performing mods we know took place on Electras, and that resulted in the configuration we know NR16020 to have used?
Yes, I think the conversation was most likely about the difference between a
10A and a 10E Special (bigger engines plus massive fuel tanks in place of passenger seats).
is there a chance the "one off" XC-35 was actually a "two off?"
When you are dealing in abstract possibilities, there is no limit to the number of XC-35s that you may construct in your imagination.
The real question is, "Is there any evidence that NR16020 was an XC-35? Is there any evidence that it was not?"
The bill of sale for NR16020 is for a non-pressurized aircraft.
What if AE and FN's route ended not being farther than planned, but higher?
To what end? Why invest in that capacity? I'm pretty sure that you don't just add the weight, cost, and complexity of a pressurized system just for the heck of it. And I doubt that the engines would perform well at higher altitudes without some super-charging (more weight, cost, and complexity). The
Wikipedia article on the "Lockheed XC-35 gives a nice overview of the differences:
"The United States Air Corps wanted the aircraft to perform high altitude research and to test the feasibility of a pressurized cabin. The Corps contracted with Lockheed Aircraft Corporation to produce the aircraft at a total cost of $112,197. The requirements called for an aircraft capable of flying at no less than 25,000 ft (7,620 m) and having an endurance of 10 hours with at least 2 hours above 25,000 ft (7,620 m). Major Carl Greene and John Younger, both structures experts who worked for the Air Corps Engineering Division at Wright Field in Ohio were responsible for the design of the pressurized cabin structure. Greene and Younger worked with Lockheed to modify a Model 10 Electra with a new fuselage consisting a circular cross section that was able to withstand up to a 10 psi differential. New, smaller windows were used to prevent a possible blowout while operating at high pressure differentials. The cabin pressurization was provided by bleeding air from the engines' turbo supercharger, the compressor outlet fed into the cabin and was controlled by the flight engineer. This system was able to maintain a cabin altitude of 12,000 ft (3,658 m) while flying at 30,000 ft (9,144 m). The fuselage was divided into two compartments, a forward pressurized compartment, and an aft unpressurized compartment. The forward compartment housed two pilots, a flight engineer, and up to two passengers. The aft compartment provided accommodations for one passenger and could only be used at low altitudes since it lacked pressurization. "The XC-35 was fitted with two Pratt & Whitney XR-1340-43 engines of 550 HP (410 kW) each compared to the two Pratt & Whitney R-985-13 engines of 450 hp (336 kW) fitted to the base Model 10 Electra. The engines featured a turbo supercharger to permit the engines to operate in the thin air at high altitudes."