69 Barracuda Side Markers
Just checked the markers on my 9ner... they are attached from the back using a 'u' shaped bracket that measures slightly longer than the marker itself.
Two coarse self threading nuts per unit pulls the marker tight against the body inset.
Not sure if gaskets were ever used originally, (I know I don't have any), but if you're conerned about the paint... you could always make some up.
Personally, I don't think you need them.