Excellent, well-researched question Dave!
Don’t lump Z/28s with BBs. Different sound, different parts.
The August 22, 1969 recall letter sent to owners is succinct. The Z/28 muffler/resonator system entered production as of May 2, 1969. The statement “…but this was a target date and subject to production stock availability” does not appear on either recall notice. The second recall states "This exhaust system will be effective in production approximately April 15, 1969." May 2nd was a Friday; your car was built several days later. N638712 was most likely built with the muffler/resonator system with aluminized tailpipes. Trust your gut!
Exhaust system configuration is listed on the Chassis Broadcast Copy [CBC], box #198. Z/28s built January 15 - April 25 have an FM code system [L48 also]. I only have two later data points; May 20 and July 2. Both have FQ code systems. The best way to nail down the date would be to find CBCs for Z/28s in the April 25 – May 20-time frame. As we know, CBCs are rarely found, even in Van Nuys cars.
There was no optional 1969 “deep-tone” exhaust system. It was a ’67 [N61] and ’68 [NF2] option. It did not have resonators and used a different muffler [#3910566] than the resonator system. Both Z/28 recall systems listed the deep-tone muffler. The second Z/28 recall lists service replacement front exhaust pipes [#3909958 & #3934871] and #3909959 resonators.
I’m convinced excessive noise under hard acceleration was the reason Chevrolet dropped 4.10 gears as a Z/28 option as of June 1969. Norwood would be shipping Camaros to CA as of August and even the resonator system apparently was still too loud for CA.
Exhaust systems are a gray area in judging. No replacement conventional exhaust system, not even NOS GM, duplicates what was used in production. In production, resonators were welded to the front exhaust pipes; the RH tail pipe was welded to the muffler. No clamps were used in those locations.
I am at an impasse with BB exhaust systems. The recall [L34 & L78] NC8 replacement parts show non-resonator front exhaust pipes #3923573 and #3912596. Vintage photos of ZL-1 #3 [built March 4th] show it was built with a production muffler/resonator system. Doug Perry’s L78 SS was built February 25th, same system. Both cars had M22s and 4.10 gears. A previous owner of ZL-1 #6 [automatic] stated it had resonators. A 12k mile survivor L78 SS built April 30th with M22 and 3.73 gears retains its original non-resonator system. There is a restored COPO built March 25 that is said to have had its original non-resonator system prior to restoration. The photos were not conclusive; resonators quickly rusted out and could have been replaced with aftermarket replacement sections. Chevrolet did not use those in production and never offered them as service parts.
The AIM is not very useful. It shows all BBs receiving a resonator system as of June. There is an original L35 car built August ’69 with a non-resonator system. The are three known CBC codes for BBs. FR was seen on an L35, probably non-resonator. FT on an L78, probably chambered. FS is most common, seen on L72, L78, ZL-1s. Even an L35 had it. Probably the resonator system.
