I think a good analogy here is an old NOS gas tank that is dull colored compared to a brand new tank that is shiny. The old tank is dull from atmospheric exposure, and the new tank looks shiny, because it is simply not old. Both were made the same way, of terne sheet (steel sheet hot dipped in tin-lead, same as solder). The NOS tank was shiny when new, but it is no longer shiny now.

"I know that the plating of today is a lot different than the plating which was done back in the 60s and 70s." AFAIK, the plating of today is done exactly the same as it was back then, except cadmium is used much less, and the handling and disposal of the hexavalent chromium found in the chromate dips and other bad stuff is done more carefully under better safety regulations.

"I have seen NOS pieces where the plating was more dull for the "gold cadmium" stuff vs. the shiner stuff of today." Although there are wide variations among batches and among vendors due to differing times, temperatures, chemicals, etc., old stuff looks old and dull from long atmospheric exposure. By and large the old stuff was just as shiny back when it was new as todays new stuff.

"Is the duller finish for the "gold cad" plated parts correct for OE or factory finish? Or do places like OE "accept" the shinier finish as that is all that is available today?" I can't say what others accept, but if you are wanting to make a car look brand spanking new and as it was when just delivered from the factory, then your gas tank and your cad or zinc plated parts should look new and shiny. If you want your car to look just like a 40-year old survivor, then the paint and everything else should be slightly dull from 40 years of atmospheric exposure.

Never heard of cadmium cyanide. You might mean chromated cadmium - cadmium plating followed by a dip in yellow rainbow di-chromate? In my limited experience with this topic, I have one original heater valve that is yellow rainbow chromated cad. It is way more eye popping and shiny than anything I have ever seen in yellow rainbow chromated zinc. (However, when I put wax on it, the eye popping luster partly went away.) From this I suspect yellow chromated cad should be quite a bit shinier than yellow chromated zinc, almost spectacularly so.