Using the tail number from the 3rd photo, 2-6323, Joe Baugher's s/n database shows it was a tanker flying the Hump from India to China. Which explains the background of the photo. The nose art on the other photos all have camels, which I assume are indications of a Hump run. The position of Item X places it underneath the cockpit floor, below the pilot's control board. The Fast Company photo shows 2 pitot tubes below and aft of the Item X patch. Engineering drawings put them directly below the pilot's knees. On the other side of the hull behind Item X run all the control surface cables, some emergency bomb door release cables, and a few other cables. So it's crowded, but if Item X is underneath the pilot's control board, there may be a fair amount of space. Since the engineering drawings don't show Item X, I assume its installation was a field modification.
Each bomb bay could hold 2 auxiliary fuel tanks, each with 640 gallons capacity. So the total capacity in the two bomb bays would be 4x640 or 2560 gallons. Avgas weight is around 6 pounds/gallon, so the bomb bay gas would weigh approximately 15,360 pounds. Within the bomb load limit. To put it in perspective, each wing of a B-29 holds about 2735 gallons of gasoline. So to fuel a B-29 in China would require 3 B-29 flights from India.
To speculate: flying a loaded B-29 over the Himalayas might have posed some problems that Item X helped resolve. These B-29s are equipped with radar, so that would be a help also. If this is a radio installation, why place it there. I would think putting it closer to the radio operator on the starboard side would be a better idea. I haven't looked for photos of these aircraft from the other side. If there is a duplicate installation on that side, maybe Item X worked in pairs.