International letter with perfin “SS” in the stamp. The stationary letter is franked correctly with 10 + 10 = 20 øre, which was the rate for international letters in the period from October 1st 1907 to January 31th 1921. The stamp is cancelled June 30th 1913 with København 1 Nr. 6a, Kjøbenhavn *I* Bro Ia-3.
Since the receiver of the letter, Professor Krause at Hydrotherapeutic University of Berlin researched in sexually transmitted viruses, I consider that the 10 øre stamp with the perfin was used correctly in the interest of the Statens Serum Institut.
The stamp is the only known sample of 10 øre with perfin “SS”