OK, let me take a whack at this.
We have to go back to the WWI era. Most radio operations were
taking place well below 1 MHz. Communication receivers we
generally crystal sets connected to large wire antennas. There
was a need for direction-finding on enemy transmitters. That
required a loop antenna, but the output was too low to work with a
crystal set or even a one-tube regen.
The solution was multi-tube amplifiers. The French had a leg up
here having produced the first mass produced tube, the "TM" valve.
See also R. J. Round and the Brit DF "B-Stations."
French amp among the Armstrong artifacts at the former Ft.
Monmouth museum.
Armstrong was working at the Division of Research and Inspection
in Paris. This was in effect an AT&T laboratory.
To get amplification at HF frequencies, where the existing tubes
didn't have much gain, he leveraged his knowledge of heterodyne
frequency conversion gained from his work with the regenerative
receivers. He built a tunable frequency converter to feed a
French amplifier operating at about 100 KHz. The circuit was
originally called the supersonic (now we say ultrasonic)
heterodyne circuit, where the incoming signal in not converted to
audio, but to an intermediate RF frequency.
Hope the pix come through.
Al