GM347 Nielen Cin. O. 37-17 (real photo)
This is a unique postcard. The second part of the number “37-17” apparently refers to the daye 1917. It demonstrates that Nielen photos were taken during the waning months of the Panama-California Exposition in 1917 (see history of “mop-up”). Nielen later published real photo postcards of Balboa Park images before and during the 1935 California Pacific Exposition. Numbers on those postcards ended with “-35”.