Here is what I've found so far:
1.) If there is no post code on the front, then it is 2004 or earlier. The post code was added between days 271 and 315 in 2004, but I haven't confirmed the exact date. This date corresponds to the change in the Uigeadail label as well.
2.) If the "bar" or date code is etched (like 2005 and later), then it is most likely 2004. 2003 had both etching and stamps with spacing.
3.) If the stamp or "bar" code has no spaces between any numbers or letters, it will be between 2002 and early 2003.
4.) If there are two rows of numbers stamped (with a double size "L"), then it should be 2001 or earlier.
This is just a quick description, but if you let me know what code you do have that is readable, and whether it is a stamp or etching, I should be able to let you know.
Also, check the small example I made below about the codes.