Hi guys. Regarding the outer rocker covers, I have seen metal rivets on 73+ original untampered with cars. I've had 4 low mileage original cars that all had metal rivets on the outer covers. Most of those cars I got from the original owners and discussed that matter with them.
I have also seen one owner late model cars (75-76) with metal rivets on the out rocker covers, other than my own. By rocker cover, I mean the piece that is painted black so there is no confusion. I discussed it with the owners, and their responses were that they never took the covers off, and that's the way the car was delivered to them.
You'll see a lot of those rivets with black paint on them due to a prior owner respraying the rocker without removing them. My light ivory 75 with 42,000 miles was like that. I ended up taking the covers off due to a bad jack point on that car. I replaced the metal with black plastic so I could easily take the covers off when needed. Actually I shaved the pins and stems so the rivet was only attached to the outer cover and not to the physical body of the car. The bottom hardware was enough to keep it secure.
With that said, I would say 73+ metal rivets for the out rocker covers. Black Plastic rivets on the later cars would certainly make it easier to maintain/clean under there. I would strongly advice against using sheet metal screws. You will deform the metal on the sill and widen the holes.
On 70-72 1/2, I can't comment/verify being I haven't seen enough original early cars that have not been tampered with/restored/modified. Pat is the original owner of his 72 and stated white plastic rivets came on his outer rocker covers. Pat has done enough concourse events with his car to know so.
Feel free to poke around my photobucket album. There should be shots of the rockers of some of the cars in there.
http://s283.photobucket.com/user/tod914/li...rt=6&page=0