I popped off my shift bezel and this is what I found (see picture below) It says adjustable electronic speedometer calibrator in small white letters. I imagine it is to correct your speedometer if it its off? Just thought I'd post a picture of it and get some thoughts if that isn't what it's used for...

 

It is a device generally used to correct the speedometer when the rear-end gear ratio has been changed.

Google "dallas mustang speed-cal" and the first hit will tell you all you need to know...