We both would love to know more about the Sensotronic Range so I invite everyone to share what they know about them and to post pics of your examples ahem Jack, Mike! lol
This Sensotronic System 20 S3450 dates from January 1988, and has only been used 3 times in it's 21 years! It's still in its original box and packaging, and wrapped in plastic.
A string of LEDs which light up as the power is raised or lowered, as well as 2 performance indicators. The green one on the left shows that the airflow is optimum. The red one on the right shows it needs to be adjusted.