Flatheads were VERY common, by lots of manufacturers in the 20s thru the 50s. Packard, Ford, GM, and Mopar built flathead sixes, straight eights, and v8s into the 50s. Overheads were rare,(mostly buick) until the caddy/olds overhead v8s of 1949. Plymouth used a flathead six thru 59, until replaced by the /6 in 60.
I have three flathead powered mopars, a six and two straight eights.
From what little I have read, back then the overhead valve motor was basically for the very high end expensive and foreign cars, till, like said, caddy/olds jumped in....then the flathead started to disappear.
