I think 74 had some disc brakes, if I remember right they were small rotors 1 year only.. 75 and up was were you found most front disc on our cars....
Seems to be a rumor out there (I have read it in some manuals as well) that the disc brakes in 1974 were different. As stated...they are NOT. No difference.
I read the same thing years ago, 74 discs were Mustang II parts, later discs were larger Granada parts.
Maybe this tech section should be edited: http://mmb.maverick.to/showthread.php?t=35957&highlight=mustang+disc+brakes