This card is one of my favorite uprated modern, surface-rate card usages. The card is ripe with markings, some of which remain a mystery.
The card was sent underpaid to Denmark during the difficult 4-Cent surface rate period. Upon arrival, the card was properly charged the postage due. The green meter marking suggests that it was claimed and this fee paid.
The purple triangular marking on the face with its corresponding postal markings on the reverse constitute the mystery. Thanx to Lars Engelbrecht who was kind enough to translate them, the latter indicate that the Valby district (of Copenhagen) post office would be holding the card until postage due was paid. Perhaps the numeric triangle was used internally at that office for the indexing of 'held' mail.
In any event, the card is modern gem of a usage.
Place mouse on image to see the reverse side of the card.