Not trying to be a technical Nancy here, but I believe the original finish was actually a belt sanded finish of about 1000 grit or finer. The type of belt they use on huge machines with long belts on soft backings, typically used to clean up cast finishes and polish parts on a production level. I say this because I have an NOS set still in the box. They are most definitely not as-cast, but have a very very fine finish to them. The finish does not go into the valve cover bolt recesses or inside the fins because the huge belts would only ride on the surface and not penetrate recessed areas. The oval air cleaners were also finished this way, but with a much coarser grit of about 40-60. The aftermarket covers are just cast. They look very nice, but that fine sanded finish looks much nicer. It's also hard to keep that way.
I've used 40 grit on a long sanding board taking full length one-way strokes to get that "just finished" look on the air cleaners, but the valve covers are much harder to do and make look right.
