Its Zavikon island, connected by a bridge to its neighbour island. It's located in the Thousand Islands, near Rockport, Ontario; 656 feet from the international border. https://en.m.wikipedia.org/wiki/Zavikon_Island
Despite the caption on the postcard, the bridge does not actually straddle the border between Canada and the United States.
Despite the caption on the postcard, the bridge does not actually straddle the border between Canada and the United States.