Need Marvel Carb Expert

[pgerhardt]

My Marvel carb mystery continues. As I started ti disassemble my Marvel Carbs to build a couple of very good ones I am still puzzled by a mystery third adjusting screw (see picture). Some of my carbs have it and others don't. On many of the sources I have found online this location is labeled "fuel bowl vent".

On the carbs without the adjusting screw there are no threads in the holes, and there is no sign that they ever had this adjustment. So maybe - yes - just a vent. But what about the ones that have this (mixture??) adjustment screw? What are they, what are they there for, and why do some marvel carbs not even have them?

I know lots of questions! Thanks in advance for your response.