Nico,

GB9833.5 indicates the production quality control standard
GB9679 indicates the 'safety' (cleanliness in the production line, etc)
quality control standard

These are not indications of year, though they can be used as a gauge to
determine the Period of production.
The wrapper inside can tell you more.
The logo on the wrapper
The design layout on the wrapper
The nos. printed on the wrapper
These are the clues, again not to a specific date, but to narrow the period
of production further

Danny