John, for a broader explanation of "--" trim tag designation, see the CRG "decoding" section.
www.camaros.org. You will see that it could mean a special paint color or stripe delete or a non-standard stripe color, but not primer only. For example, my '68Z built in May of 1968 is a "--" paint code and it is tuxedo black which was dropped in January of 1968 as a standard color.