Sorry about the delay - here's the picture.
As you can see, the PO didn't remove it when he repainted the car, so it has some (blue) overspray on it.
I looked in the '68 AIM under RPO U73. It doesn't call out the plug.
The grommet that was used with a fender-mounted antenna is shown as POA of the cable assembly (RPO U63, sheet B1).
Hope this helps.