Did all the 418 covers have a dull finish?
I forgot to chime in on the first part of the question - my originals and all of the unrestored cars I have ever seen had "mill" finish cosmetics - they are smooth, but not polished. IMO bead blasting ruins the finish - about the only way I have ever seen as an effective way to clean them is dip them in carb cleaner, but you have to be carefull with that as well (some cleaners are so aggressive they can open up or pit the surface). True, they get worse with age and oxidation, but polishing them is not correct, nor is bead blasting. I will have Jerry restore mine when I get to the point where I need them done.
I sold a set of mid-80's NOS dripper covers a while back - they were polished covers, and you could really see the heat cracks were showing up in the casting dies. Shortly after I bought them, GM superceded the part number again, making that the third part number away from the original 418's that I know of. BTW, as you know, both original L&R covers did have the same casting number, as any number of posts will mention. Took me years to get word on that - I thought somebody had replaced one of mine with a duplicate side for a long time.