I have a photo of a friends recall letter issued Aug 22 1969 by Chevrolet which states VERBATIM
"Dear Camaro Owner,
1969 Camaros equipped with engine option Z28 (290 h.p.), L-34 (350 h.p.), or L-78 (375 h.p.) produced prior to December 4, 1968, incorporated a chambered type exhaust system. 1969 Camaros with the engine option Z28 (290 h.p.) produced from December 4, 1968 until May 2, 1969 incorporated a regular muffler dual exhaust without resonators. It has been determined that these exhaust systems may exceed the state of California maximum noise level during instances of maximum acceleration in the lower gear ratios."
If he grants permission I will post a copy of the letter.