When the 100th anniversary of Aston Martin was celebrated in July with a display at London’s Kensington Gardens, much of the focus was on cars ranging from the earliest production models to the DBR1 that won at Le Mans in 1959—not to mention the DB5 of James Bond fame. But perhaps the most underappreciated aspect of Aston’s history is its long association with Italian coachbuilders, most notably the Turin-based design house Zagato, which unveiled two all-new customized Astons commissioned specifically to comme…