The "card" appears to be a magazine cutout, likely from an advertisement. The stock and the fact that you can flip up one of the corners enough to see what appears to be text from an article on the back lead us to this conclusion. Envelope is postmarked Jun 12, 1939 Cooperstown New York in conjunction with basbeall's centennial celebration and bears the Centennial of Baseball 3¢ stamp with First Day of Issue Cancellation