James,
I checked and the Thule Society contributed the swastika. This society was founded towards the end of World War I, just as the German revolution was beginning. It was an anti-semitic pan-german sect that was named after the supposed location of ultimate Aryan purity, Iceland, and used the swastika to denote its racial priorities.
What was interesting about the Thule Society is that it contained several members who were to be prominent in the Nazi Party such as Hans Frank, Rudolf Hess, Alfred Rosenberg and Dietrich Eckart. One of its newspapers, the Volkischer Beobachter (the Racial Observer), was eventually purchased by the Nazi Party. It was from this society that a member made contact with Anton Drexler, who founded the Nazi Party. So, this group was quite influential in the formation of the Nazi Party.