Ok looked at around 130 examples and with eliminating repaints and some rattle can jobs came up with some findings.
Please bare with me as I go through this and consider this is just what I can glean from what I have available.
Challenge I had is that going through the survey I got mixed results until I remembered the hood black out discussion we had on the site a few weeks ago and looked that those pictures which in some fo the NJ examples showed the cowl color.
So going back through the cars I kept four columns. Non Mach with and without black and Mach with and without black. With this a pattern emerged and shown below. Reminder - this is only what I found and is limited to what resources I have available at this time. Invite others with unrestored 69 Metuchen examples to share so we can see if the pattern is supported or shows something new.
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69 NJ (non-Mach I) - examples with body color cowl (panel above the firewall to cowl pinch weld that the cowl cover is attached over)
9T102871 - 9T151587
69 NJ (non-Mach I) - examples with black engine compartment paint over body color cowl body color application.
9T1774xx - 9T210602
All of the Mach I examples I found had black applied to the upper cowl above the pinch weld. 9T12323x (also have pictures dated for a car built 10/28/68) -
9T215xxx
Below is a side by side comparison (by VIN range) from two cars one non- Mach the second a Mach I. Did this for the earlier period to illustrate what I found.
9T110000-9T120000Mach I
9T120000-9T130000Mach I
9T130000-9T135000Mach I