The option over the years has been that they were painted. Never had a NOS from the period to confirm and the used ones I did have appeared to be painted. You could rub, scrap or samd through a section and see the difference
The Boss and CJ sites have a fair number of threads on the subject and suggest include
- Pinstriping tape over the rib top edge then paint scoop black and peel back carefully the tape when done and before paint fully (black dries)
- Paint the scoop black. Get a fine texture trim paint roller and apply the silver using the roller to the ribs
- Purchase a Testor's paint felt tip pen and after painting the scoop black apply silver paint the the top surface of each rib with equal pressure that the the felt tip is not pressed down and around the side edges
Sure there are more options that I've not heard of and of course its easier just to sand but be careful of the filings/residue as it can make a mess of things. At least it has for me on things like valve covers and aluminum topped air cleaners
Just one of a number of threads
https://www.428cobrajet.org/forum/index.php?topic=21064.msg141406#msg141406
https://www.boss302.com/smf/index.php?topic=3293.msg20405#msg20405