I believe the big block manifolds were entirely painted and the paint burned off on the hotter areas leaving paint around the bolted areas. It actually just flakes off rather than burning. Here is a quote from John Hinkley on another discussion. http://www.camaros.org/forum/index.php?topic=9898.0
"Your photos look fine to me. Iron-intake big-blocks didn't get any masking, and the bypass hose was pretty much completely painted; aluminum-intake big-blocks did get a mask, and their bypass hoses will show somewhat less paint than on an iron-intake car. Tonawanda painted the big-block engine with the exhaust manifolds in place, and they had lots of paint, as shown by the photo on the Camaro assembly line. Flint V-8 (small-blocks), on the other hand, installed the exhaust manifolds AFTER engine painting starting in 1965, so post-'64 Flint-built small-block exhaust manifolds won't show any paint.
Attached is a photo from 1967 assembly line showing a full size car with BB with manifolds completely painted. George