Cut off date for Chambered Exhaust

[Mr70]

This should help.
From Technical Service Bulletin 69-T-8 dated April 15th,1969.
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"The sections of this bulletin that apply to Camaros with RPO Z/28 are being revised to include a letter design exhaust system.

Many of the early production 1969 Super Sport Chevelle and Camaro models were equipped with a chambered exhaust system.There are some local and State police Departments issuing warnings and citations to the owners of these vehicles for what they term "excessive noise".
Consequently,and effective in production on November 25th,1968,the Chambered exhaust system was discontinued as the standard exhaust system on Camaros w/options Z/28,L-34 and L-78,and also on Chevelles with options L-34,L-78 & L-89.
Since that time,the standard exhaust system on these vehicles has been the regular muffler dual exhaust as used w/RPO L-35,(325HP),except for the Camaro Z/28.This used the same regular muffler dual exhaust as option L-48,(300HP).
The chambered exhaust system is still available as optional equipment known as RPO NC8 in those area where this system is permissible.
The latest design system for Camaros with RPO Z/28 option includes resonators ahead of a modified deep tone muffler to further reduce the noise level.This system will be effective in production approximately April 15,1969.
On the basis of a customer request,Camaros w/RPO Z/28 option with the early built chambered,or the interim built regular muffler dual system,should be converted to the latest exhaust system w/resonators.The remaining options that used a chambered system should be converted to a regular muffler dual exhaust system.The parts required for conversion are listed on the following pages of this bulletin."
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It then goes on to show diagrams and part numbers to make these conversions at all Chevrolet dealerships on vehicles previously mentioned.