Pretty sure mine were as you mentioned either unfinished or zinc plated.
Looked at the truck listed right now in the ebay section which looks like a survivor truck and they appear to have been black.
So, Greg, you have an example for both ways.
Are there body-colored repair plates/panels near the filler plates?
Non-original valve cover, with interesting PCV system on that 1954 engine in that eBay ad?
Anyone else with a 1954-1955st?
[I'll try to add images below - for that eBay 1954]
