Built 1928~1933, probably in Turin.
A supercharged four-cylinder 500cc engine with dual overhead camshaft and water cooling built by Valerio Riva made a brief appearance at the 1928 Monza Grand Prix. It exploded on the first lap. Later another 500cc 4 cylinder machine appeared with front suspension which is understood have been supplied by George Roy of Majestic fame. An image of the 30s machine in the Milani book is titled 1930, but a forum post says the rear hub is by Baruffaldi which was not made until 1933. The machine is also briefly mentioned in Le mythe des 4 Cilindri by Claude Reynaud.
From the motoclub-tingavert.it thread:
uomo francese mi ha detto che ha fatto un'intervista con Georges Roy. Georges Roys è stato l'inventore del francese Majestic. Roy gli ha detto in questa intervista che in trent'anni Valerio Riva fargli visita e acquistare la sospensione anteriore di Majestic. E Georges Roys dare questa foto per l'uomo che ha fatto un'intervista per fargli vedere quello che Riva ha fatto con la sua sospensione.
So, it would appear that the image of the Riva on this page dated 1933 was given to George Roy by Valerio Riva.
After considerable discussion on the origin of the engine, Gérard Martin writes:
Riva was working on projects until 1939 when war broke out.
There was also a Riva made in France in the 1950s: Riva RSI