Yes, thats exactly how it was done, the input signal was mixed with a 3.455 MHz oscillator, a low pass filter used to remove the higher mixing product, the counter chip then? read the difference.
Other Wadley loop receivers of that era used a similar arrangement, the Century 21 being one example. This method was a very simple and elegant way to get an offset? using analog? mixing, rather than digital divisors, and, being in a seperate box with its own power supply removed the dangers of any birdies and interference being introduced into the radio..
The display chip itself is simply the manufacturers data sheet implementation of that chip, so a schematic really is not required to get this working once again.
