Small oval Porcelain U S A Sign

Need help; This is a very old embossed Black and white circle and letters, I S A porcelain Sign. Size 4 3/4 wide by 7 1/4 long. Been trying to research this item for 2 days what it was use for , date, and value very rare antique if you can help solve the problem be great. Thanks jim Gas Station, auto exc.