Your S code GTA came from the factory with a 120MPH speedo like all other Mustangs of the year. Only Shelbys came stock with the 140MPH speedo in 1967. 1965 GTs and all 1966 (even the T code 6 cylinders) had a 140 MPH speedo.
Sometimes the original literature showed what could have been or what designers intended to be... I have an article from 1966 previewing the 1967 Mustangs that shows the "wood" steering wheel without the crash pod in the center--only a month before the cars were released (with the new, government mandated crash pod). They had contined the 1966 "wood" wheel in the pictures.
Thanks Daniel, I am sure that my car has the original speedometer (120mph) But when I saw the Mustang recognition guide with the picture of the 140mph the question came to my mind. I had a 1965 GT that came with the 140mph speedometer. Thanks for clearing my mind.