Thid said Esteban to me (or I'm Esteban)
:
The detector has an audio gen. tone with speaker, first models with transistorized flip-flop as tone gen.
Why BFO? In early times this was common detectors. So, later we use IB, off-resonance and primary experiment with PI. And other kinds no-based on regular MD.
For today is enough! Go and work!