Danish postman Einar Holboll conceived the idea of issuing a tuberculosis fundraising seal at Christmas time. TB funds exist earlier, the first, 1897 was issued in Germany for a sanatorium on Lake Grabow (Green's #291). Two old articles, below, contain loads of interesting details about Denmark's first campaign.
I have seen very few Denmark #1 1904 tied ons, and they normally reach a price over two hundred dollars at auction. The used seal, off cover is common, and I have seen over a thousand, usually with a readable date, December 1904. Mint seals are scarcer and more desirable than cancelled ones. The high value of the seal tied on is because few covers survived. In that era of collecting, the value was placed on the stamps (or seals) which were soaked off cover and tied up in bundles of 100.
holboll 11-47 coronet article.pdf 1904 denmark article.pdf