This 'model year' confusion has been going on for years.

In the US all new models are released in their Autumn (Fall) around September. Mid year models are released in Mar/April (like the 64 1/2 Mustang).

The GM model year begins on September 1, although this sometimes varies if a new model needs to be released earlier or later than this date, for production scheduling reasons.

What Byron is trying to get across is that HZ Sandmans produced between Sept 1 & Dec 31 in 1977, are considered by GM to be 1978 'Year Models'. Likewise those produced between Sept 1 & Dec 31 1978 are called 1979 models. The sixth letter in the VIN will tell which model year it is.

It's commonplace in modern cars to see things for example like, MY09 even when the car was actually built in 2008.

So we have to differentiate between the car's 'Model Year' (MY) & when it was actually built, they often differ.

Dr Terry