[William]

It's not when it was ordered that matters it is when the car was built. Chambered could have been ordered as late as May. The letter, effective May 19, states "...all orders specifying this option [NC8] will be produced with regular dual exhausts."

The last cars built with NC8 as an option were likely in the N645xxx-N646xxx range, last date of production May 16, 1969. The last 05A cars are in the low N644xxx range so chambered was possible.

The May 19 date is mentioned twice in the letter as it was the date chrome tailpipes became standard with dual exhausts. So the weekend prior, all the now-obsolete parts were likely removed from the assembly plants. Chrome tailpipes included a cost increase and that had to be coordinated with production.