Georges Ronsse was an accomplished Belgian cyclist with a hatful of victories to his name, notably Paris-Roubaix in 1927 and Liège–Bastogne–Liège in 1925.
Most remarkable is his record in the World Road Race Championships - which he won twice, in 1928 and 1929. In 1928, in Budapest, he won by a mere 19 minutes and 43 seconds - the largest winning margin ever recorded in this event. This photo comes from the 1929 Worlds, Georges Ronsse is the rider leading the group.
I think that he won both his Worlds titles riding fixed - so did not use a derailleur. You can just about convince yourself of this from looking at this photo.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)