Manufactured 1926-1927
Franco Tosi began building diesel engines around 1907, and developed a thriving business. Tosi partnered with Emilio Bozzi to build Wolsit motorcycles, and at some point one of the Maserati brothers was working with the Tosi firm.
Franco Tosi Meccanica went on to become a major force in Italian industry, producing ships, submarines and large steam turbines. The company had 6000 employees in the 1970s. It was bought by one of the Castiglioni brothers, owners at the time of Ducati.
Sources: wikipedia.org, Cycle World, Mauro Columbo.