Portuguese number plates: numbers in yellow field?

Anyone driving a car in Portugal will ask themselves what the numbers in the yellow field on the right mean: These are the month and year of the first registration!

The top number the year and the lower number the month the first registration. Our Audi A5 in the photo below was therefore first registered in October 2016. However, the yellow field in which the first registration is written has only existed on Portuguese license plates since 1998.

Otherwise give the number plates in Portugal no information about origin or region. The numbers themselves are random number-letter combinations from which no origin can be read. Only until 1993 did the license plates bear abbreviations for the corresponding region.