Phosphate

05A 12k miles car....Joe

Thanks for posting Joe, to clarify, shown in the photo are the standard, short style bumperettes vs the optional, long style.

Short. You're welcome....Joe

The lower bumper guard Bolts, I have seen two Styles. Some cars have a recessed head with a captured lock washer. But more cars than not have the captured flat washer. The application for the bumper guard should be zinc chromate. However this is the same exact bolt that attaches the starter brace to the block. And those were black phosphate. So it's possible that they got switched up at the factory on occasion or if they ran out they snag some from that bin

Thanks Rich. I also reached out to "JM" for his input on the question and he responded: "Silver Zinc, indented hex head with a floating washer. Standard and full rear guards used the same bolts"

I just got other pictures from another known original 69 z/28. Phosphate, exactly like to 12k original car....Joe

Here's a sampling of headmarkings for this application.