Heavy Duty Manual Steering Option N38 was available in (at least) '66 and '67.
How was it different from standard manual steering, and can it be visually identified on the car?

 

HD has 5.33 turns lock to lock. regular manual steering box has 6.25 turns.

 

Thought that a quicker box might be part of the equation, but what makes it "heavy duty"? And can it be externally identified?

 

afaik, the only difference is the internal ratio. A 66 box would have the cast iron cover plate; late boxes have aluminum. When off the car, you will get a little more lock-to-lock than when installed, maybe 5.5 turns or so with the HD box.