As far as I know, the formats are:
-from around 1999 - 2004: 8 digits with the first 2 indicating year. 99xxxxxx = 1999, 01xxxxxx = 2001
-starting around 2005: 9 digits with the first 4 indicating year. 2005xxxxx = 2005
-recent MII owners report 10 digits: NHJxxxxxxx We haven't decoded this one yet, although the last 2 digits may indicate the year. NHJxxxxx11 = 2011 (?)
-from around 1999 - 2004: 8 digits with the first 2 indicating year. 99xxxxxx = 1999, 01xxxxxx = 2001
-starting around 2005: 9 digits with the first 4 indicating year. 2005xxxxx = 2005
-recent MII owners report 10 digits: NHJxxxxxxx We haven't decoded this one yet, although the last 2 digits may indicate the year. NHJxxxxx11 = 2011 (?)