Chambered Exahaust, RPO NC3, was available from the factory until, I believe February or March, of '69. Local vehicle safety codes made law enforcement types frown on this "straight pipe" system, so GM removed it as an RPO because of mounting pressure. However, it was still available as an "over the counter" high performance accessory at your local Chevy dealer.
Are you sure it's the Chevelle chambered system thats available? I've seen the Camaro systems available for years, but never have I seen a repro Chevelle system. I have the address of a guy in Michigan who makes 442 chambered exhaust systems. Maybe these things are available. Any responses?
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