The successor of Nagra III was (obviously) the Nagra IV. It came in 3 different versions: L, D, and 4.2.  Before the IV-S, this was THE giant leap technically-wise: separate recording and playback heads, built-in monitoring speaker switchable to source or tape, reference generator, modulometer, alarm indicator for power supply, and tape transport. 

It also has high-pass filters for recording or playback and switchable automatic level control and limiter. Thanks to this, Stefan Kudelski entered the family of Oscar winners for his scientific and technical achievements.