The green faced Grand Prix's are the older model (pre fender) and the red face (or black with red stripe and chrome logo) are closer to the fender buyout. I'm unsure of when the newer screening process started so I can't give you a year that they changed it.
The guts are pretty much the same though. Some of the resistor tolerances might be a bit looser in the newer model as a cost saving measure on Fender's part...
