Help! I think I screwed up. Timing issue.

[Yewhi]

Hi all. Well I messed up. I mistakenly turned the knob on the distributor in the picture below just below the distributor cap.



Immediately knew I made a mistake and attempted to reposition the knob in it's original position. Truck started fine but then I took for a quick spin around the block the engine began running progressively more rough. Got it back to the drive way and began backing it off a quarter turn at a time to with only moderate improvement.

A few questions...

1) What is this knob? Is it related to the vacuum advance? How do I adjust it?

2) Obviously, this is a timing issue but for the life of me I don't understand how to correct it. Any help is greatly appreciated!

Thanks in advance,

Tim (feeling like a complete idiot)

ps. It is a '55 series 1 with a stock 235.