Given the exact wording of the Testi headbadge, I believe there is a very good likelihood that the world champion who rode a Testi bike to victory in 1947 would be Arnaldo Benfenati. I come to this conclusion based upon a number of logical deductions: The rider would need to have been an Italian amateur (as opposed to a pro) with geographic links to Testi. Were he a pro, he would have ridden one of the pro team bikes unless he was an independant (which no period Italian pro world champion was) The geographic link is also very important as at the time, immediately after the end of the war, travel was very rare and news from other parts of the country were slow getting around. Riders were brought up by local sports clubs, supported by local framebuilders. You would not travel hundreds of km as an amateur to get a frame, as you wouldn't need to do so to get a frame (virtually all towns had a framebuilder) and framebuilders reputations were almost always regional, not national. I therefore checked out all of the world champions that came from Italy:
Coppi (it is well documented that he rode Bianchi at that time), Ferrari (who had no geographic ties to Bologna as he came from Cremona province which would have linked him to framebuilders from Lombardy before Emilian builders that were further away and not as well-known), Guido Messina (who also had no geographic ties to the Emilia region and as a 17 year old could not possibly have attracted the attention to warrant sponsorship from so far away), Mario Ghella (I have reasonably dependable information to indicate that he rode a Benotto, which would also jibe with the fact that he came from the Province of Turin where Benotto was based)
This leaves Benfenati, who was an amateur world champion and participated in both the 1947 and 1948 worlds. He won the 1947 4 km pursuit world championship. In 1948, he would have raced as reigning champion. He also came from the province of Bologna, the home of Testi bikes. The sole oddity is that when he passed among the pro ranks in 1949, he passed with the cross-town rival of Testi: Cicli Cimatti.
-- Steven Maasland Moorestown, NJ
PS: To find all the world champions' names I checked them up here: http://sportsfacts.net/