In 1969, for Chevelles, the last production date was the 3nd of June... I know becuase I was planning to order one, and missed the date by a few days.. I think the end of June was *normal* changeover time during those years for all models. Back then the new cars were introduced generally in September... of course, the end of '69 Camaro production (and Corvettes also) was very different in '69 for several reasons...
Gary