Anzani complete motorized bike, dated 1921-1922. 75cc four-stroke OHV engine is water-cooled, while standard Anzani engines were air-cooled. Possibly this is a prototype that never went into production.
Anzani was an engine manufacturer founded in 1907 by Alessandro Anzani (1877–1956), which produced proprietary engines for aircraft, cars, boats, and motorcycles in factories in Britain, France and Italy.
"In 1921, Anzani continuing his experimentations, thought of motorising bicycles. To this end he built a small, 4 stroke engine, 75cc capacity, with belt drive. These small engines were easily fitted to normal bicycles and could run for 70-75 km. on a litre of petrol. Anzani, therefore was the precursor in the production of cycle engines which would corner the market in the post-war period" (from britishanzani.co.uk)