The M bodies came along about 4 years after the first F body showed up. Chrysler had more money, and 4 years to correct a few issues. Harder rubber mounts, thicker upper control arm brackets with some support, and bigger sway bars. Even the later F bodies and the J bodies in between the early F & the M were better handling cars then the early F bodies were.
The F body came out in 76, the M body came out in 77. Mechanically the same, cosmetically different.
You might be thinking of the J body which came out in 80, again, mechanically the same, cosmetically different.
As the mechanical improved they were implemented across the platforms. F/J/M.
F body, the original K car.
