A guess; It is a Stamp club postcard with a place for the local Postmaster to sign So maybe they were told to take it to the local Post Office, having affixed their favorite stamp to get it post stamped for keeping. A Stamp Club thing to do. Forrest Lake MN sounds like a town you could do this in. Notice how carefully the card is hand stamped so as to just touch the stamp. It is - in my opinion - A personal thing.