Some further detail. I went through my parts stash and pulled a fender patch and spare vent (small bracket was sold off). In the picture you can clearly see the spot welds (one has pulled thru). Second picture shows vent in place and where bracket would match-up.
https://www.flickr.com/photos/advance-design_parts_co/34838095834/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/35678935655/in/album-72157631868452754/
The two holes drilled in the vent had occurred on a few trucks I have seen/owned. It seems it was common to mount a two post antenna here and then make the short under dash run to a radio.