Thanks guys.
Yes, I can see in U15-A1 that, it is the speed warning device which appears to use only one of the notches for the mounting bolt and the adjustment knob is on the device's bracket. Thank you!
However, the headlamp grounding connection UPC12-B2 is sandwiched between the instrument carrier's mounting screw and the ground wire is screwed into the dash bracket, so it doesn't use them.
Maybe the second notch and hole is a provision for an accessory that wasn't required..