The US Library of Congress is a superb source of historical photographs on all sorts of subjects. Some of the most remarkable images are from rural areas in the US in the 1920s and 1930s. All the Library of Congress images are in the public domain.
Here is a small collection of pictures of farm dogs from the US in the 1930s.
Here is an example. This looks like a robust, working style Rough Collie type of dog to me.