You all are confusing two different issues. All automakers have at one time or another released cars in mid-year. The most famous is probably the 64 1/2 Mustang, but there are many, many others. As automakers have gotten away from the traditional September release of new cars, this has become more confusing but less of an issue.
As for an 87 GN with an 88 build date, that is not legally possible. Automakers are forbidden under federal law from building older design cars in later model years. The requirements on safety, emissions, etc. all require very specific certification for the specific model year. If an automaker were to build a 1987 model car (for example) with an 87 VIN in 1988, that's absolutely against federal law.
Automakers ARE allowed to build newer cars early, however. It is perfectly legal to certify the car to newer and thus more stringent requirements but then sell it in an earlier year. One example that comes to mind is the 1993 Cadillac Allante. Production of the 93 models started in April 1992 (admittedly so the Northstar-powered version could be pace car of the 1992 Indy 500). Same thing happened with the Corvette. There was no 1983 Corvette (thereby missing out on a 30th anniversary issue). Due to the cost of certifying the new C3 models, GM went straight to a 1984 version, but sold it in 1983 also.
