'S' was used for GTX, it had a lot of the same parts as the 'P' Satellite, seats, grill, tail lights.
AFAIK, fender tags can be reproduced with the same codes as the original tag IF you have a broadcast sheet or a copy of the IBM card (available from Chrysler Historical). Otherwise you're just guessing at some of the codes and you don't have real proof how the car was built.
If you're thinking of purchasing a 67 GTX, just know they have a lot of trim that is costly to restore and rear quarter panels are not available.