Some states/maybe all states use the compliance plate date as the 'make date' for their uninformed paper jockeys. NSW RTA inspectors need to supply RTA with both make date and compliance plate date on application.
You are lucky you have both plates affixed.
Ordering parts genuine will not be a problem. Non gen parts books are often a balls up, insisting Brumbies had EA82 and that the timing belt kit you bought says suits EA82 Brumby ! Those of us in the know can laugh
Think a moment, to release the new 2012 models early in the year all tooling has to be made years before so the cars can be made in 2011 to be ready for January sales in 2012.
If the Brumby had changes coming up you would have a MY1991 but it was made in 1990. You have a little problem when ordering stuff? What stuff?
Some Brumbies are said to have 1994 compliance plate dates but manufactured in their last year 1992. Not sure where compliance plates are or were affixed now or then, whether it was at the docks, wholesaler or at the dealer ?