In the days when auto license plates were free-standing there was space to bolt on a message — akin to bumper stickers and window decals. This one promotes Claremont, California. Auto dealers, aftermarket auto products/services and charities were some of the subjects of other toppers. The 1939 California license plate also has text promoting the Golden Gate International Exposition.
Vintage tag toppers are rare and collectible. A few for sale can be seen here: http://www.oldplateguy.com/License-Plate-Frame-Toppers-For-Sale/Original-Vintage-Toppers/