Below are links to photos of a "Tone Standard" (my term).  I have
no idea what this would be used for.  Perhaps in some laboratory
setting?

Regards & stay safe,

Richard Cardwell
Palo Alto, California

 [   front
 [ https://www.mmdigest.com/Attachments/20/08/26/200826_001735_IMG_2773.jpg
 
 [   side
 [ https://www.mmdigest.com/Attachments/20/08/26/200826_001735_IMG_2774.jpg
 
 [   dial close-up
 [ https://www.mmdigest.com/Attachments/20/08/26/200826_001735_IMG_2775.jpg