Who was Joe Kusstatscher and how did he take pictures of the Nagasaki Bomb Detonation?

One of the most famous pictures that exists from the detonation of Fat Man over Nagasaki is accredited to Joe Kusstatscher, who was apparently a photographer with the US Navy. See Truman Library for source.  Unfortunately, Mr. Kusstatscher is not officially reported to have been a member of any crew of Bockscar or any of the observation planes that flew with the mission.

Fay Millward has always claimed to have been a photographer assigned to both the Enola Gay and the Bockscar.  Fay claims that he was given verbal orders to fly on both missions by W. S. Parsons. See the letter from W. S. Parsons below.  However, like Kusstatscher, Fay is also not recognized by any official source to have been on either flight.