Part of your air injection system for the catalytic converter. Same as used some of the later 70s -82 Corvettes and some Chevy vehicles!
Valve, air injection reactor check valve, to keep air from blowing back from the manifold!
here is a listing in one of my corvette parts suppliers! https://corvetteparts.com/item/valve-air-injection-reactor-check-replacement-1968-1982