[William]

The 2nd design has a vent tube with 2 45º bends and a raised bead stamped in the base to elevate or better seal the element. Chevy began using it in production during the model year; June-July or so. Some service units were provided with chrome lids. Every service ducted hood air cleaner I have seen is the 2nd design; never heard of an NOS flat bottom.

I do not know what motivated Chevy to change the design but the #4296 carb issue is a plausible theory. The change was done to accomodate a Holley double-pumper carb and that was the only application to use one.