The L36s from 1966 through 1969 were painted; chrome only came on those when they were installed in 1967 through 1969 (Impala) SS-427s.
The 1967 L72 is like a unicorn. Officially they didn't make or install them.
L72s in 1966 were painted.
I will say one thing; C3 Corvettes confuse me as to chrome on the 427s. I would expect chrome on L72 Corvettes and their triple-carbed spinoff, the L71, plus the L36 and it's triple-carbed spinoff, the L68. Observations don't clarify things as it seems to be inconsistent. Corvettes are a different animal.