I believe your observation on the 67 and 68 cars is spot on. I believe the zinc plated horn bolts were superceded to dark phosphate around the early run of 69 Cheverolets. That has just been my observation from looking at unmolested cars.
Most 69's hand very few zinc plated fasteners under the hood. Exceptions being some of the sheetmetal screws like VR, horn relay, ground straps, and washer bottle bracket hold down screws.
I am not at home now and can not give you the page #'s and PN's now. But in my two coppies of the 69 Camaro Assembly manual where it shows one of the horns and bolt detail, there is a revision to that bolt, dated I believe 10/68. In the lower revision box on that page it states the # of the zinc fastener and the word "Was". I believe on Camaros they started using the same dark phosphate bolt that holds the lower hood latch to the core support and header. Not serated, and with a captured washer.
I had asked Steve Shauger this same question awhile back concerning horn bolts on 69 Camaros. I believe he told me that he had seen Zinc horn bolts on some early 69's only.
Always take the time to soak in all the detail stuff on unrestored cars, not just Camaros.