Chevrolet built about 120,000 Impala Super Sports for 1966, but only a sliver of those left the factory pairing the 425-horsepower L72 427 big block with a four-speed manual instead of the far more common automatic. It is a combination so rare that even dedicated Bow Tie historians cannot agree on an exact production count, hidden inside a car that looked like nothing more than a comfortable family cruiser. Read on to find out what made this pairing so scarce, and why so few original buyers dared to check that box.