This envelope arrived from Sioux Falls, South Dakota. I've spent a decent amount of time in Sioux Falls over the course of my life, as my maternal great grandmothers both resided in South Dakota, one in Sioux Falls and one in Centerville. I always liked the city. As it is, these are the first cards sent from South Dakota, and as Walter's note below indicates, they're not ordinary cards.
That's right, these are "errors." One blank front, and two wrong backs. Thank you for the cards Walter, they are very much appreciated.