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My Carburetor journey.

Longer duration cams run at higher idle speeds....which means more idle air. Hole could have been enlarged because a bigger cam was fitted.....
I do not think it is a hole for a HIC because [ a] the surface is rounded & there does not appear to be any where to mount the HIC.
Another thing I found odd on the Six Barrel set up was the metering plate on the rear carb.
The fuel inlet holes were plugged with screws and above them two new holes were drilled?

Those are the main system metering holes, equivalent to main jets in a metering block. Maybe some body wanted to reduce the size & this is how they did it?