*In General Terms: I think it was plant (Flint -vs- Tonawanda) & year (1959 -vs- 1969) of mfr. dependent.
So I have seen a vintage photo 69 Nova w/ fully painted manifolds.,,,,but the real question is How that sampling really relates to Your Restoration.
A Review of the Engine Mfr machine and Dress articles (Reports) are in order to understand the answer.
awe shoot Links ??
Look at the .crg list of research topics