This undated postcard, published by Zenith Interstate Company of Duluth, with Plastichrome by Colourpicture Publishers of Boston, appears to be circa the early 1960s.

The caption on the back reads:

A Panoramic View of Duluth, Minnesota
The Harbor at Duluth-Superior is the second largest harbor in the world in total tonnage handled annually. More than 10,000 ships arrive and depart annually from the Duluth-Superior Harbor.