You would probably have to contact the original breeder (if you knew who that was). They will usually have a way of deciphering what it says. Also the breeder could have bought way too many leg rings and used them for a long time after the original date.
Makena's leg ring says "Aloha, HI 573 '07". The breeder said the Aloha, HI signifies the bird was bred in Hawaii, the 573 was the birds ID while at the breeder, and the 07 indicates the year he was born. As far as I know there is no standard for leg rings. Good luck and I hope you are able to find out!