On August 25, 1988, the USPS issued a set of 5 25-cent stamps that commemorated 5 different American classic cars. Those are: 1928 Locomobile, 1931 Cord, 1932 Packard, 1930 Pierce Arrow, and 1935 Duesenberg.

These 5 stamps are on 3 covers cacheted by Artcraft for the Postal Commemoraive Society, and are canceled with the First Day cancel from Detroit on August 25, 1988. All are unaddressed. 

Check the scan for details.