If that's what this is, ^^THIS^^. you had a device inside with a transducer at each end. Essentially, the radio fed one transducer (think speaker) which was hooked to one end of this spring. The sound traveled down the spring, delayed by the length of it. At the other end, the vibrating spring transferred the motion into the second transducer (think microphone) which converted back to electrical "sound." This then went to a simple amplifier, which hooked to the rear speaker(s).
Many of them SOUNDED exactly like what they were............a screen door spring