First welcome to the site.
For your answer you might try the search feature above. There are many threads discussing the "natural" heat treated look for the rear and front springs and how to maintain/protect them.
The library also has an article titled
- Natural Finishes- Reproducing that should answer some of your questions
As far as the rearend itself. the main housing is painted semi-gloss black with a unpainted surface/face where the third member was attached. Retaining studs are dark bare steel as are the backing plates to rear end housing studs for your application.
As far as "MCA Approved" There are no "approved" methods and we don't tend to slant our answers here on a particular organization unless the question is posted in the MCA (for example) section of the site. More interested in what is/was original and what works as both should no negative effects when showing pretty much anywhere
Again welcome to the site - hope you find the information and experiences here helpful in your endeavours