Sure you've done the basics by comparing the VIN on the car, buck tag and door tag. Just setting down a foundation with the basics
Interesting and it would be helpful to see the buck tag also since you report that it is a third color code.
But agree it is a bit of a puzzle
If we look at the Library and the color chips shown there for 67 and 68 we see the following
1967 - 8 = Spring Time Yellow
1968 - W = Meadowlark Yellow
But unlike some other years these appear to be two different colors. Not just a marketing thing were a name is changed to make it "new"
1967 Spring Time Yellow = Ditzler 81510
1968 Meadowlark Yellow = Ditzler 81584
Likely very similar but different.
Two digit color codes on Mustangs typically include a "5" indicating a hood or taillight panel paint option. But no "5" in this case and never seen an "8" used for this purpose.
Other observations. No six digit DSO to suggest a possible special promotion in the Atlanta area, can't think of any nor would they typically have then in that part of the sales year. Those are often done in the spring to push the car line once the sales slump after the first 6 or so months after the start of the new model year.
Need to look at other Dearborn built Mustangs built around the same time to see if one of the operators was entering 67 or 68 or even more two digit paint codes before speculating any further IMHO
