From the Wikipedia Article -

On the morning of September 13, 1948, Free raised the American motorcycle speed record by riding the very first Vincent HRD (it is debated as to whether it was a Black Lightning or Black Shadow), owned by the California sportsman John Edgar and sponsored by Mobil Oil, to a speed of 150.313 mph (241.905 km/h). Special features included the first-ever Vincent use of a rear shock absorber, the first Mk II racing cams, and horizontally-mounted racing carbs. Free had already developed a style of removing the seat from his mount, and lying flat prone along the back spine – thereby minimizing wind resistance, and placing most weight over the rear wheel.