Intake manifolds are interesting, and I do think the finish depends on the casting process that was used. A new Blue Thunder is certainly different from a new Edelbrock. The FE intake on my website is an older Blue Thunder that has been vapor blasted. The original 1935 Ford flathead intake that I have here is a beautiful casting with a nice sheen and was fortunate enough to have been preserved with 80 years of oil and dirt. The aftermarket Evans intake I have is probably from the 1950s. The casting quality is not nearly as good as the Ford original, plus it has been blasted who knows how many times, so it has taken some work to get it to have a newish look, but it will never look as good as the grease caked Ford part does. Last week I did. 1979 L82 Corvette intake for a local guy. It was untouched but came out with a different finish than the ones mentioned above.
I guess the point of all this is that in my opinion there is no one right finish, but there should be a subtle mixture of dull and sparkle in a new sand cast finish. The finish on diecast parts is of course different and has a good variation of colors that can disappear very easily with buffing or bead blasting.
Also, unless persevered in a vacuum or some other sort of oxygen free environment, metals are going to oxidize and dull over time, so 40-50 year old NOS parts can possibly be a bit misleading. There’s no reason to believe that an alternator housing on a new 1969 Mustang should have a different look and luster than that of a 2019 Mustang. Anyway, my offer stands to do a test piece for anyone here and let them decide if it’s what you are looking for.