My saved pics of mainly 77-79 era show these two type of screw cap and rivets very early on.
https://www.78ta.com/Pages/images/1970cowl.jpg (1970)
https://www.78ta.com/Sliders/images/secowltag.jpg (1979, replaced tag but clear shot of plain screws)
https://www.hitmantransam.com/Pages/images/1977cowl.jpg (1977)