The trim tag was the first part produced for the dealer order. At some point it was attached to the body during fabrication, prior to any coatings. The [blind] rivets were installed with a device that expanded and ruptured them. They were filled with seam sealer to prevent engine bay fumes from leaking into the ventilation system.
The firewall was not extensively primed; I have never seen primer on a body tag. One of the last body fabrication steps was firewall blackout, a skim coat of satin black paint. This usually included the entire tag although it is not unusual to see little to no paint at the top. Also not unusual to see light body color overspray across the top.
There are a few exceptions to this. Some '67 tags were masked during firewall blackout, no paint at all. Some were painted white after installation, then masked. A very few early '68 tags were painted white off the car, then installed and masked.
Excellent question!
