Maybe it depended what they had on hand, so some got 660's and others got 661's late in the production run.
__________________
Rick
69 Chevelle SS396 Silver L78 M21 Post Sedan (Berger Chev.)
69 Chevelle SS396 Black L78 4-speed 4.10 Post Sedan
Don't text and drive.
Somebody's life may depend on it!