It gets tricky when you realize that the 1987 Grand Nationals were built into December of 1987- Technically they were 1988 cars but were still titled as 1987 cars.
You then have to go by the sequence number in order to determine if your car was built in, as an example, November of 1986 or November of 1987. Both would still be 1987 model-year GN's by Buick standards.