From davnews.com (edited):
License plate keychain tags began use in 1938. The New York-based “Lost Key Services” was among the first companies to produce them. The Disabled American Veterans (DAV) purchased the Idento-Tag company in 1941. The DAV was able to persuade the various state motor vehicle departments to co-operate with their requests to obtain mailing lists of the people who registered their cars. The DAV mailed tags with registrants’ license plate numbers on them and asked for a donation. The premise was that if you lost your keys, the finder would drop them into a mail box, and the DAV would return them to you on their dime. The program continued from 1941 through 1975, when it was ended because of a “right to privacy” rule.