Years ago I had heard that there was a problem with casting porosity on a batch of iron 6bbl manifolds. It appears this affected some (not all) cars built early-November to early-December. Right when the bulk of the Superbirds were being assembled. Aluminium manifolds were used to keep the engine assembly line moving.

My former C02 (12/2/69) Fury GT has an iron manifold. (it was built about 2 weeks later)

rftroy, What plant was your RR assembled at? I'd bet Lynch Rd.