Diatto Tipo Unico (12/15–18 CV) History

After buying out French partner Adolphe Clément-Bayard in 1909 and renaming the company Autocostruzioni Diatto, the brothers introduced their first fully independent design: the 2.9-litre 16/20 T-head four. The subsequent L-head 12/15 of 1910 — sometimes called the 'Fréjus' — became the make's staple prewar model, a well-engineered touring car produced at roughly 250–300 units per year.

By 1912 the engine had grown to 2,413 cc (the '18 CV Unique Type'), equipped with a four-speed gearbox and available in torpedo tourer, cabriolet, spider, and limousine body styles. The basic platform continued in production through World War I (during which Diatto also manufactured aircraft engines under Bugatti licence) and remained on sale until 1922, when the entirely new Tipo 20 replaced it.