The year that is about to close marks two noteworthy and related centennials. In 1907, America’s greatest sculptor, Augustus Saint-Gaudens, died. Also in that year, the federal government issued the gold coins — in $10 and $20 denominations — that it had commissioned Saint-Gaudens to design. Most people know of Saint-Gaudens for his large-scale public works that ennoble certain lucky American cities, including and especially New York. But as a fine exhibition mounted by the American Numismatic Society at the Federal Reserve Bank of New York attests, the master sculptor was equally adept on a scale as small as a coin.