I received this reply from Kepich when I inquired about their resonators:
"I have never seen the resonator sold separately. It was orginally welded into the tailpipe from GM. I have a set from a low mileage car that I use as a pattern for over 30 years. As far as the grooves go, GM never made an exhaust system. They purchased them from a supplier, generally there were about 10 of them. As the car was assembled, it could have had one in a number of mufflers from these various suppliers. The ones with the grooves are AP brand, now merged with Goerlich & Maremont. I am surprised that ones with grooves came on the car, because GM would've specified that the muffler be numbered or stamped with a GM number. The grooved would've indicated AP, as the grooves are their trademark. Thanks, John"