Well guys, I was directed to Mark Ellis of the 1969 CORVAIR GROUP
and he stated that basically, the "D" is an indicator of the Interior Paint Color. On the Trim Tag (I'll use mine as an example), you have an Exterior Color Code ("53" Glacier Blue), an Exterior Soft Top Color ("A" White), an Interior Fabric Type and Color ("704" Metallic Dark Blue Coated Fabric) and the Interior Paint Code ("D" Blue). This told the Fisher Body workers what color to paint not only the outside of the car, but the inside. So mystery solved, my car was assembled the 1st week of December, 1968.
After Mark and I talked, I found another site called CORVAIRKID
that shows this exact information for the 66 model year!