Although I swore on another thread that I'd no longer help anyone out, I'll answer your question.
S-197 is the Ford internal program # for the 2005 Mustang project. S = Sporty Car or Small Car, depending on who you're taking to. 195 is simply a sequential program #. Only a small # of vehicle programs actually make it to production.
Mustang fanatics tend to refer to the different generation Mustangs by their program #s so people know which model year Mustangs they are referring to without having to spell all them out.
The last generation Mustang, 1999-2004 had the program designation SN-95. It looks like S-197 will apply to the 2005 - 2008 Mustangs as the 2009 is rumored to be getting a major update. If true the 2009 Stang will carry a new program #.
Go ahead StangNut, tell me how I'm wrong.