Help to read date code

[bitwelder]

Hello,

yesterday I went to buy a few 74hc4050 from the local shop,
and when I later I examined them I tried to find when they have been produced (just for fun).



I was expecting something on the line of the old YYWW (year-week) scheme, so the gobbledygook on this chip was quite a surprise.
As much as I could follow TI datasheet, two possible 7-symbols schemes could apply, a UZXYYTT-type (from NatSem) or a YMLLLLS one, but with neither I was able to make sense of the letters.
I only got the final E4 as lead finishing (= preplated leads, lead free).

Can somebody help to decode it please (possibly with pointers to TI datasheet table) ?

[fluxcapacitor]

Heres a few pointers that will help you decode it.

http://www.ti.com/litv/pdf/szza020c

http://focus.ti.com/quality/docs/gencontent.tsp?templateId=5909&navigationId=12626&contentId=5071