The year of issue in tiny lettering included in the image of a stamp.  This first appeared in Australia on the Sports stamps issued on 13th February 1989.  Additional year codes were also included in most Type 3 reprints from 26th June 2007.  (See Reprint – Type 3.)

(Source:  Australian Stamp Variations Catalogue : Decimal 1966 – 2009.  www.asv.net.au)