My first thought is that this item was a cut square from an old collection. It is on very stiff, thick paper though not what I would call card. It is embossed. It had a gridded cancel similar to other postage with the central number 15 in the cancel.
What actually threw me a bit is that the back of the stamp is gummed. Some disturbance, obviously, since it is used, but the entire back was gummed. Then, when I compared the face value to other issues, it seems to be a relatively high denomination stamp and not what you usually find on postal stationery. Can someone with a specialized catalogue please enlighten me?