"to achieve the 50 mph (much less 60 mph)...a cadence well in excess of 300 rpm given any stock rings available at the time"
Actually, with a 53 chainring and an 11 cog, 50mph is achieved at 130 rpm (assuming a 68cm diameter wheel, which is 26.8 inch or 2.23ft; circumference is then 7ft; 53/11 is a gear ratio of 4.8, so we get 33.6 ft/pedal revolution. Then 33.6ft/rev X 130 rev/min = 4368 ft/min = 0.83 miles/min = 49.8 mph). This is not surprising since racers often pedal down mountain passes at such high speeds.
Brad Stockwell
Palo Alto