I think a few companies made them under different names, but the one that comes to mind first is the Vortex Impeller. I think it was made by JMJ? Someone correct me if I'm wrong
My oldschool PBN Feedback +48 Positive/ZERO negative : before the days of this "fancy" feedback system.
"When people get a little too chummy with me
I like to call them by the wrong name
to let them know I donít really care about them."