After finding our house, I guessed it was built in the 1920-1930s. After googling things like "American Architecture of the 1930s", I finally found it:
The American Foursquare:
According to this, the American Foursquare is also known as a "Prairie Box" or the clunkily-worded "Transitional Pyramid". It was a popular mail-order style, hence the above ad. Everything would be delivered in a box car with a book of directions and all the parts pre-cut and numbered for self-assembly. Sounds like fun, right? No wonder they were only $1,995.00.
I love the reminder at the bottom of the ad: DO NOT ATTEMPT BUILDING WITHOUT PLANS