I have one of these plates and had never seen another until your post. I used a mild rubbing compound (jewelers rouge) to clean off the accumulated grime. It is pretty soft alloy.
Something else l did since I did not want to drill my dash. I attached the plate and screw heads with acrylic caulk (non-silicone). If you need to remove it peels right off and did not damage my paint.
https://www.flickr.com/photos/advance-design_parts_co/8129178187/in/album-72157631867396207/
This is the parts truck I pulled mine from (1952 3600 Farm Truck).
https://www.flickr.com/photos/advance-design_parts_co/15218113173/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/15837526685/in/album-72157631868452754/