This is a tuned radio frequency (TRF) receiver, so it has no local oscillator to convert the tuned frequency into an IF. A signal generator is simply a variable frequency oscillator, so injecting the sig gen into the antenna connector at 1400KHz is the same as receiving a radio signal at that frequency.
The radio is very simple: a signal is amplified in the first two valves, then the audio is extracted by rectification and filtering out the RF. Be aware of course that the chassis is live to mains, so great care needs to be taken if working on the set while it is powered up.
By the way, I had difficulty reading the diagrams you sent, and had to get the originals from RMOrg: you should use a higher resolution in your pictures! 🙂