According to Mannell (Mustang and Fords Small Block V8), page 7-19, picture 7D4, "Beginning around 4/5/67, all California 289 HiPos were required to have T/E, so Ford switched to new HiPo exhaust manifolds." The date is mandatory if built after, but it does not say, refer or imply when Ford actually implemented the change to the new smog manifold. It takes months from "rumor", to "mandate", to "design", to "prototype", to "test", to "production", to "comply" with a date that fluctuates due to "politics". California was very "confused" about smog in those days. (California law states that cars built/sold after January 1st, 1966 shall have smog, yet all 1966 California bound Fords, Mustangs included, were equipped with smog equipment from production day 1, August 16, 1965. My 66 GT K Fastback Oct. build, San Jose, was "exempt". Supposedly because of a "limited production" model car.)
Jim