Pullies
 

I have a question of no great importance but I was curious. I have noticed on 1964 big cars that some of the engines (both 389 & 421) have double groove pullies on the alternator and PS pump and are driven by a pair of belts. On other engines the alt. & ps only have single groove pullies. These cars I know are original and unchanged. Was the double groove set up some kind of heavy duty option?