The operation of the MA4204 can be heard in these examples of input, scrambling and descrambling.

This is a sample of the the audio prior to scrambling.
This is a sample of the audio which has been scrambled.
This is a sample of the original audio which has been descrambled.

As one can hear, the descrambled voice quality is quite reasonable.  The sender's voice can be recognized, and no authentication is required.

The MA4204 audio encryption device inserts itself between the microphone/ handset  receptacles  (Audio1 or Audio 2) and the output plug to the receiver.  (Photos via E-bay)

Above and below: Internal views of the MA4204.

Nameplate sample.

All photos in this table via E-bay

MA968B battery box for MA4204. Top view

MA968B battery box for MA4204. Inside view

MA968B battery box for MA4204. Bottom view

Nov 30/17