Ah yes, the weekly what is that thing on top of the 707 pylon thread.
It's a turbocompressor. Essential for aircraft pressurization.
If you look at the entire Boeing/KC135 series you'll see a variety of diffent configurations. Some have 3 turbocompressors, some 2, others 1, etc.
"There’s nothing quite as exhilarating as being shot at and missed" --Winston Churchill