Considering the relative simplicity of this transmitter circuitry the receiver would probably have been comparably simple. Could be two resonance bridges (two c

Considering the relative simplicity of this transmitter circuitry the receiver would probably have been comparably simple. Could be two resonance bridges (two capacitors and four resistors each) tuned to the two reference frequencies respectively. A simple rectifying of the bridge outputs would give a differential pulse sequence which would be fed to a comparator. From there it would be bit detection and synchronization as is done in a typical UART.