(Maybe reversed values here but)Normal de-emphasis is set at what's referred to as 75us (microseconds). It has been from day one of FM transmission. It was always there but invisible to the user.
When Dolby FM attempted to make inroads, it required it be changed to 25us to maximize it's usefulness. That's when the switches started appearing on FM tuners and the confusion started.
Since Dolby FM pretty much went down the same path as quad, I doubt any tuners made since the late 70's still incorporate this switch.
