year of crates

I know I'm beating a dead horse but, could you please tell me how to find out what year a 604 is?

right front corner of the block-----just above the water punp

(2) rows of numbers
1st---1MXXXX......that is the production code

2nd--XPO4.......this is the date code----whatever the "X" is will be the year

example:
8PO4 is a 2008
9PO4 is a 2009
0PO4 is a 2010
1PO4 is a 2011

Thank you.....