I have a 72 Nova with a cowl tag with no paint code. Where the paint code would normally appear (right side, second line) are the characters “- B” indicating a black vinyl top (which it had). The car appears to have been painted black, but I thought black was not a factory color for 72?

The dash shows it was painted a color not available for that year/model. If it was black from the factory then you would not have had a stamped code on the trim tag.