That was their unit insignia until just before the outbreak of war.

Taken from Wikipedia:
The division's original shoulder sleeve insignia, approved in August 1924, featured a swastika, a common Native American symbol, as a tribute to the Southwestern United Statesicon region which had a large population of Native Americans. However, with the rise of the Nazi Party in Germanyicon, with its infamous swastika symbol, the 45th Division stopped using the insignia. Following a long process of submissions for new designs, a new shoulder sleeve insignia, designed by a Carnegie, Oklahoma native named Woody Big Bow, featuring the Thunderbird, another Native American symbol, was approved in 1939.