I'm talking about the bottom surface looking up. Prior to 1970 the surface was flat, all over the counter bases after that had a "Rib" around the base indicating it was a later design. the 1966 427/425 original air cleaner I had was a base that was flat with no rib going around the diameter, they all had a ridge to keep the air filter element (A212CW) in place. The same goes for the Cowl Induction Bases for 1969 going forward. but Hey, that's what I remember having had many over the years and sold both types.
Bill
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