Freeways/expressways around me are 45 mph minimum. They are normally mounted below the limit sign.
https://www.twincities.com/2015/05/20/more-than-700-miles-of-interstates-in-wisconsin-to-be-70-mph/
But I have seen small tack on signs that are the same in most regards.
https://commons.m.wikimedia.org/wiki/File:SPEED_LIMIT_75mph_MINIMUM_SPEED_50mph_NO_TOLERANCE.png
Tuning forks!!! The Zenith clicker The buttons would work strikers that would hit tuned rods. A different one doing a different function.