By the way I finally looked at the video. Sam unscrews the mouthpiece, which he discovers is designed for easy servicing. An attempt to unscrew the earphone is less successful.
At this point my ancient decrepitude comes in handy. When I was a child I would read books about how things worked. It turns out that phone mouthpieces and earpieces were traditionally of quite different design. The earpiece was a quite ordinary moving coil miniature speaker, but the transducer in the mouthpiece was made up of graphite (carbon) particles packed tightly together. This was done because, unlike electromagnetic transducers, it could give you a strong enough signal to pass some distance down the line without electronic amplification.
So Sam might want to take a closer look at those mouthpiece transducers. Could they be carbon microphones?
I recall also that banging the mouthpiece on a table top was thought to loosen the particles in a dodgy receiver.
Edit: 1942? That would be during the occupation. Depending on geographic origin, this factory might have been under German or Vichy control.