I’m convinced that this was a supplier thing. I think that sometime in 1965 Chevrolet started getting the 15-dimple lids from who ever was stamping them. It’s not just on the chrome lids. I have seen painted lids, with a single, tapered snorkel and 15 dimples. Another thing that I have noticed is that the lower edge of the lids is different from a chrome lid vs.a non-chrome lid. Chrome lids are not rolled shut like a painted lid. I think to drain when they were pulled from the plating tank. The chrome lids are both mine. Left is my original 63 340 hp lid and the other is an original, 16-dimple, 400hp lid. The painted one is an original 61 single snorkel lid.
Ross