I always thought is was a auto/4spd thing Both my late 4426's have the single adjuster While my mid 68 4401 had two adjusters.
Yea, I'm still trying to figure out exactly what the deal was on the adjusters, I can only go by used originals I've seen and the VERY few surviver cars I've seen with untouched carbs. I've had a couple pretty knowledgable people suggest that maybe all of the 68/69 carbs originally came with single adjuster but were later modified to have the 2 lower adjusters, however I've seen a fair number of double adjuster carbs that looked very correct and original, so I feel pretty certain they came both ways.
