X3! Having published guidelines would be wonderful but I'm guessing that the guidelines would be huge due to all the variations that could be seen at the two plants and variations due to some processes being done by hand. The variations seem huge to me. I have a Norwood car with a gray primer underbody and another has a Norwood car made 3 months earlier that has a mostly black underbody.
For restoration, I have been using Jerry's 69 book and his "Appendix E: CORRECT COMPONENT COLOR AND PLATING CHART".
One problem is that things change over the years and what is acceptable one year, might not be acceptable the next year. A drastic example.....I'm pretty sure that undercarriages could be either black or gray back in the early 90's.
Below is a picture from Camaro Enthusiast May-June 1993. The title of the article is “UnderCarriage Detailing Component Finishes” by Frank Serafine.
Don't make your undercarriage gray though. Jerry's book (and every restored Concours car I saw at the Nationals) has "30-percent gloss black paint".