What does 'Product Date' refer to on VAG parts?

[TangoFiver]

Am looking for a NOx sensor on eBay, and on the photos of the parts, under product date it says:

40 / 08 L2 (off a 2004 Golf)

and

13 / 13 L2 (off a 2005 Golf)

How does that translate into the Gregorian calendar? :-s

 

[Darren92]

Week & year

 

[TangoFiver]

Thats what I wanted to know - thank you sir!