Based on the uniform on the guy in the back I am going to say German officers. American serviceman have always used pins to distinguish rank, Germans used patches and if you look close you can clearly see change on the collar. I don't know how much significance it really is but the breast pocket doesn't look like an American uniform either, they are usually quite tight to the jacket and not very big, while after a little research it does look like some German uniforms had larger loose pockets.
