1957 Buick Estate Wagon Dash Question

What is the number 20 on the lf. bottom of the instrument cluster? Looks like a revolving readout? Cruise? Speedminder?



https://www.hemmings.com/classifieds/cars-for-sale/buick/unspecified/2259061.html#&gid=1&pid=4

 

That's the speed when the (nanny) buzzer will sound off.

 

Amazing! Speed minder back in '57. These cars are too cool.

 

One of my fav 57 color combo’s, usually seen in reverse of this one