My glass was bought new from Amazon, serial number 21 08200147, bought in late 2007.
If the Canon 10 digit coding was in effect then, it would imply an Aug 2006 manufacture.
The serial number marker (thank you, Binastro) between the tubes is only a strip of plastic held in a small recess. In my case, it has faded almost completely. A magnifier with a strong light can still barely allow the 147 to be discerned. Fortunately I also kept the box cover with the serial number.
Guess Canon is no different from Zeiss, they too had serial numbers on bits that could easily go missing...