I always thought the tags were held on with 1 screw and bent up so that the underside(tag and inner fender) would get some paint on it. If there wasn't any primer/paint under the tag, it would start rusting immediately.
My T/A was original paint when I bought it in '78 and my tag was a bit rusty on one side to the extent that one screw wasn't holding it on that side. I think the subject has come up before and either the tag was installed after prime but before paint so there's primer under the tag not paint, or it is painted under the tag. Sadly I didn't know to document anything on the car before I had some bodywork and repaint done in '81!
