single vs. double field alternator?

seems like back in my swap meet parts huntin sellin days, findin what we thought was 100 amp cop alt, i don't have handy everyone's resto catalogue s, but classic industries show a 100 amp alt used on s/6 70-76 2 groove pulley, small block 70-76, 70-73 , thats 318 and 360, 70-71 383 etc....