I don't think I posted this yet (I did check), but here is a picture of the date code I found on the back side of my headliner. This is an overhead console care which accounts for the shading.
Aug 66 compared to Dec 66 build date. Light blue as part of the two tone blue interior.
Not sure what the T E X represents.
There were no other markings.
While I'm at it, I can document the headliner bow colors for my fastback:
1. Front - purple
2. Middle - green
3. Rear - pink