This card was used as a notice of the availability of new technical and scientific publications which were specifically relevant to Mining.
The card looks to have possibly been sent 3rd class sometime in early 1921 (No Date Slug). Unlike many other official cards, this one does not indicate a form number nor a printing edition size. Perhaps this may have been a "one-off" printing job at the GPO with a very low quantity of cards printed and distributed.
Place mouse on image to see the reverse side of the card.
Click this link to see another Department of Interior card from the Bureau of Pensions.