G is 1977 model year (Sept 76 to Aug 77). A is 1981 model year, so it follows that G would be 1987 model year too. Not sure after that.

There are several anomalies in the GM year codes. Here is a full list.

1971 – A
1972 – B
1973 – C
1974 – D
1975 – E
1976 – F
1977 – G
1978 – H
1979 – J
1980 – K
1981 – A
1982 – B
1983 – C
1984 – D
1985 – E
1986 – F
1987 – G
1988 – H & J
1989 – K
1990 – L
1991 – M
1992 – N
1993 – P
1994 – R or Q
1995 – S
1996 – T
1997 – V
1998 – W
1999 – X
2000 – Y
2001 – 1
2002 – 2
2003 – 3
2004 – 4
2005 – 5
2006 – 6
2007 – 7
2008 – 8
2009 – 9
2010 – A
2011 – B
2012 – C
2013 – D
2014 – E
2015 – F
2016 – G
2017 – H
2018 - J

They appear to be quite logical up to the dual code in 1988, as commodorenut has mentioned, but then in 1994 for some reason they used either R & Q depending on the car. Then in 1997 they jumped U & went straight to V.

For 2001 they missed Z & went numerical up to 2009 = 9.

Then in 2010 they went back to A, B, C etc. However the A wasn't equal to year 1, as it was back in 1971 & 1981.

Also in the later cars, from around 2002 onwards, the year wasn't defined as at 1st Sept. The MY approach was used. For example when the VZ was released (Aug 2004) they were all MY05 even though were probably built in June/July 2004.

The VZ 2006 update (5L387835 onwards - Jan 2006) this changed to MY06, so the old Sept 1st was no longer used.

During VE, each time there was an update whether it be in May or Oct or whenever, the year code changed.

Dr Terry