Three plants built Mustangs - San Jose - R, Metuchen - T and Dearborn - F. San Jose stopped Mustang production prior to the 71 model year. Metuchen stopped Mustang production on Dec 23, 1970 (my Mach was built 12/22/70) and Dearborn was the sole Mustang plant from then until the end of 73 model year production.
Dearborn did build Cougars concurrently with Mustangs, as noted with the 5xxxxx unit numbers.
Lorain, Ohio - H, and Atlanta, GA - A, built the Torino & Montego in 71, so what one may think to be and "F" might actually be an "H".