Hello,
I just bought a carb off of someone here on the board. And its slightly different then mine in one regard. Can you guys please help explain what the difference between the two circled areas are?

I realize one carb the holes are sealed and not pass through, where the other does...but what does this do?

thanks
Jeremy

That's for the hot idle compensator, a thermostatic valve that opens when hot to admit extra air at idle, thus increasing the idle speed, which increases fan speed and water circulation, which brings down underhood temperatures. It was present on BBS and 1920 slant-6 carbs in '70-'72 (some applications, not all) but had disappeared by '73 or so, probably a victim of cost-cutting. Don't attempt to drill the passages and activate the HIC on a carb not originally so equipped even if it has the partial casting as your carb on the right does.

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thank you Dan