Factory Small Block Heads
I believe you are both right.
Although open chamber heads had been used decades before the pollution era, I think RRR was referring to chrysler ditching the closed chamber design heads in favor of the open chamber heads at the start of the pollution era as an attempt to lower the compression ratio and hence lower NOx output. I should say I'll fated attempt, aa NOx went down, so the carbon monoxide and hydrocarbon levels went up and performance went down.