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91-96 have the "traditional" cooler ports. 97+ is when they moved to center lubed. 91-93 can "easily" be converted to 94+ by changing the EPC solenoid, drilling a hole in the valve body and blocking a hole on the separator plate (or just replacing the whole plate). 91-93 also had a different case connector but the majority have been upgraded by now as the early plug was leak prone.
Far as I know, the cooler return port change happened mid-year 1997. Early '97 had the cooler ports at the front, later '97 had the return port moved to the rear.
My '97 has the rearward port. I don't know what month the change happened.