The card, serial #4360/1, was printed in the late 1920's by Ross Verlag, a German company. The front features an attractive portrait of Louise Brooks issued by Paramount, her American studio. Their studio stamp is in the lower right hand corner. What is especially telling is the back of the card. The stamp on the back is from Latvia (prior to WWII a Baltic nation sandwiched between Lithuania and Estonia). The person who sent the card dated it June 28, 1931.
Interestingly, the postage suggests that these German cards were circulated around Europe. And, that person who bought the card was in all likelihood familiar with the subject of the card; that suggests that at least some of Louise Brooks' films were show in the Baltic states, and that she was at least something of a known personality. Perhaps Brooks was featured in a movie magazine, or in a newspaper article? I would be willing to bet she was "known" because of her three European films, Pandora's Box (Germany, 1929), Diary of a Lost Girl (Germany, 1929), and Prix de Beaute (France, 1930).
Also interesting is the date. By 1931, Brooks' career was beginning to falter. In the United States, she appeared in three lesser films, one of them a short. In the European arena, she hadn't appeared in a film in a year. And yet, a fan in Latvia sent a postcard with her image. Can anyone translate the inscription on the back?