12A is prior to the implementation of X codes on body tags at Fisher.
N555679 was final-assembled on or about Friday Dec 6th meaning it was ordered late October/early November. U16 and ZL2 didn't exist at that time; cars built with D80 used the '68 rear spoiler. Straight head restraints were discontinued mid-November. COPO 9737 included the 140 speedometer; was n/a on the Z/28 and also didn't exist when the car was ordered.
You be the judge.