Production number was my first thought when trying to figure out the numbers on the labels, but I bowed to the knowledge of the Selskab group, as they've people a lot better versed in the past running of the factory.
But some of the earlier labels (Kastrup-Holmegaard and before) have similar numbering systems on them and these don't seem to refer to year codes.
This label could, plausibly, be a year code - '42' - it's from an early boxed set of 1962 'Skotland' glasses:
(http://www.holmegaardresource.com/images/label_15.gif)
But this one is marked '01' - and there's definitely no zero month or zero quarter

(http://www.holmegaardresource.com/images/label_4.gif)