Turbo...
generator?
Close, but no cigar.
The correct term is turbocompressor.
The JT-3 and JT-4 engines fitted to the 'ole 707 (forgetting for a moment the RR Conway, altho the same applied here as well) supplied engine bleed air to power the turbocompressors, and these, in turn, supplied compressed air to pressurize the cabin.
Most models of 707 had only three turbocompressors, fitted to engines 2,3,4.
Number one engine normally did not have one, so the pylon looks different.
Some may indeed ask...why use turbocompressors when bleed air is available from the engines?
The short answer is....
The engine bleed air tended to be a bit oiley and, under engine idle conditions, there was not enough engine bleed air to properly keep the aeroplane pressurized and allow the cabin altitude to be reduced for descent/landing.
The turbocompressors were more efficient, and therefore used to advantage.
B707...a classic machine. I flew in Command on this aircraft for eight years and truly found it most enjoyable.