Ever heard of Ionia Manufacturing? Turns out, they made a lot of important car bodies and body parts from 49 to 96. Thanks to Andrew F for tipping me to the story on Mac's Motor City Garage!

Under a variety of names and owners, including Ypsilanti Reed, Mitchell, Mitchell-Bentley, and A.O. Smith, the Ionia works produced automobile bodies, components, and finished vehicles for virtually all the Detroit automakers. Between 1949 and 1964, all Buick station wagon bodies were made here, more than 140,000 of them, and Olds and Pontiac wagon bodies, too. Corvette and Continental Mark II bodies were also built at Ionia, and the factory also served as the final assembly plant for Shelby Mustangs from 1968 through 1970.

https://www.macsmotorcitygarage.com/video-the-ionia-body-story/