good grief
it was AAA 000A when I was a kid, the last letter was the year. When they got to Y (because Z looks too much like 2?) it became A000 AAA with the first letter indicating the year. When they reached the end of the alphabet again it became AA00 AAA with 00 indicating the year, 00 for the first half of 2000 and 50 for the second.
It's not tricky to work out, it's obvious enough in most cases. Is it weird that I know that from looking at number plates and not from having it explained to me?