These eventually morphed into what were later called QSL cards.
In the mid-1920s the first AM stations were just getting established. At the same time, large radio corporations started experimenting with shortwave stations. The hams using shortwave had a goot notion of who was listening via return transmissions. But these BCB stations never knew who was listening.
This buisness need was filled with something called the applause card. As the name imples, an applause card was a card that conveyed a message of appreciation by US mail. (Actually, shortwave applause cards came from other nations as well) Applause cards were in common use in the years around 1923 to 1926.
Applause cards were basically postcards with pre-printed lines for certain programs and other station specific data. Some cards also contained space to list the listeners equipment and reception quality. There were many sources of applause cards. Some listeners printed their own. Some companies printed generic applause cards. a number of radio equipment companies were known to use them for promotional purposes.
great peice here: http://www181.pair.com/otsw/AppCards.html