Having one of the 2 cables I decided to copy it to make myself a second one.
There is 6 distinct wires per cable and I think it shouldn't be too difficult to find a similar cable.

Servo-sound cable

But the proprietary connections (male and female) need to be reverse engineered.
Both connections are strictly identical and made of 4 different parts that shouldn't be difficult to 3D print.

Servo-sound connection

So I started by designing the 4 parts back from the original cable using SketchUp.
This was quiet easy and pretty fast to do :

Servo-sound connectionServo-sound connection

Next step will be to print them and check if they can fit together. But I'm confident they will.
ATM I just don't have the time to print them but I will soon show you how it's goin' on... and will tell you how this legendary vintage hifi sounds like.

To be continued...