So how a C9 exhaust tip got into the Aug. 68 MPC ?
As soon as it was determined to not "carry over" the C7 or C8 tip, and a C9 part went through the design engineering process, in time to get through manufacturing in time for the first 69's to roll off the line. I would guess that August or September of 68 would be reasonable for that.
As a parallel activity, all of the new stuff would be put together into the first release of the model year 1969 MPC. A date of August 68 also is very reasonable.
If something breaks or needs service in Sept of 68 for a 1969 car bought in Aug 68 the dealer needed the MPC to find it.
It would really be best if the MPC were in the hands of the dealer before the first 1969 cars arrived.