Doug, I believe for many years the underneath was a red primer. What year is your car?
Many, many years ago I purchased my 1973, 1975 and 1978 TA's all brand new. I know for a fact the 73 was just red primer and I believe the 75 was also red primer. I don't recall on my 78, the 1980 was black, but as you can see form the pictures, much of it was covered in the body color white with overspray.
Likely some one at TAC knows when they changed from red primer to body color overspray. I will see what I can come up with.