What did this come off?

Well, some of them (not sure of the year changeover) had a choke well that was open to the exhaust crossover, and you had to use those little metal "bathtub" inserts with a gasket to seal them up.
They were a PITA, as the gaskets would burn out or the insert would get a rust hole and you'd be pulling your hair out trying to figure out where that da** exhaust leak was coming from... who thinks to look next to the carb?
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At some point they enclosed the choke well, and the insert was no longer needed. And eventually they went to the late "open sided" choke.
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Yeah, '70-72 had that goofy design....and not just on small blocks. Slants and BB's had them too.