Here's an example of a scarce usage on a modern-era postal card.
The use of the 'library cards' by public libraries was probably winding down by the time this issue came along. There probably were still plenty of the revalued 1-Cent cards available anyway. The card was supposed to be issued during the 'war tax' period alongside it's similar 1-Cent cousin issue. The most common proper usages for this card appear to be expiration notices from Western Stamp Collector magazine to subscribers.
Evidently, the address provided on this card caused some confusion with PO officials as seen by the auxiliary address correction markings on the face. Interestingly, the card was backstamped on new year's eve and 'received' on new year's day 1957.
Place mouse on image to see the reverse side of the card.