I believe that device you have circled is what they call an over-run valve.

It leans the mixture by introducing air into the mix when the engine is decelerating with some load, like going down a grade still in gear with your foot of the gas pedal.

I believe they abandoned that idea on later carbs of the same model, they just left the casting boss on the side of the carb un-machined.

Just check to see if there is an open hole inside, if it's one that was abandoned it will be blank.

Chucky MoMo <img src="/forums/images/graemlins/wink.gif" alt="" />