hmmm..... all I could find was that a prototype 250GT Cabriolet given to race driver Peter Collins in 1957 was fitted with disc brakes as a test by the driver himself and that was the first ever, though only an experiment. Soon after, Ferrari converted themselves a racing Testa Rossa to experiment more thoroughly but I cannot find when was the system officially implemented... (possibly the '59-'62 production 250GT Cabriolet w/discs was the first street car)