The catalog editors seem to recognize that the UX27C card is slightly scarcer than the UX27D card in used condition, but I believe it much more difficult to find than this.
This one was used by the New York Public Library as notice for an overdue book. The fact that a public library came into a supply of the emergency paper cards illustrates what prolific users of postal cards these institutions were.
Place mouse on image to see the reverse side of the card.