The three liter Tipo 401 motor of 1920, was Fiat’s first twin-cam engine. Starting in 1920 the type numbers of Fiat top-category racing cars began with 801 and their correspnding engine with 401. “This complicated engine ran only briefly and in prototype form in the T.801 racing car”.
Source: Griffith Borgeson : The Classic Twin-cam engine, p.130.
The colorpic is of a 1921 car with non-desmo motor to give you an idea how this Fiat looked like in those days.
N.b. : “SD” in the listing means “Stirrup Desmodromic”.
[No exact pic available yet]
[Classification = Group 3]