I just measured mine. 0.105 so 3/32 0.097 or 7/64 0.109

Can't imagine what would provoke a claim that these are "junk carbs" or that the car is "supposed to" have a Holley 1920. Neither is a correct guess.

For future reference, applicable carb tag number is 3840s. Floats may be obsolete from some or all of the superficial catalogues, but they're still easy to get. Usually not needed; they don't tend to fail.

This what's plugged with a screw is the vacuum balance port. Some carbs have it and others don't. That screw isn't supposed to be there; remove it and hope the screw hasn't enlarged the hole.

(it's not a distributor vacuum advance fitting, that's on the other side of the carburetor, protruding from the middle (not lower) casting. The red arrow pointing to where the distributor is hooked up is, in fact, the distributor vacuum advance port.)

This carb, like most of them this age, will need more than just a clean-and-gasket-kit job; looks like it's overdue for the throttle body to be rebushed. That takes a specialist or an expensive special tool not worth purchasing if you're not going into business.

Pics in service manual are generic, not specific enough to transform an uninformed guess into sound advice.* The hole is supposed to be there. It was not put there by a shadetree mechanic. It was put there at the carburetor factory. Yes, it's really supposed to be open to the atmosphere. Changing the size of the hole or selectively plugging it is into the realm of advanced carburetor tweaking and tuning, not a good idea to wade into until the basic carb is in as close as possible to perfect unmodified working order.

*Ohhhhhhh yeah…now I remember why y'all don't see much of me on here any more.

To this day I've never heard a plausible explanation on what was the decision maker on what car got what and why.

Only difference was the Holley car had a 170, the Carter was a 225 car. But we know that's not the decider as plenty of 225s had the Holley and 170s did run the Carter.

Does anyone have any info for why and when this was tuned and for what parameter?

I can only guess it is for modifying idle transition

or balance vacuum for a multi-carb setup

and why so many of these do not have this hole

(And don't anyone get in wad...this is not a challenge; it is a question to better my/everyone's knowledge.)

Think. The problem isn't in asking a question to better your knowledge. The problem is in handing out "Oh yeah, I know what I'm talking about, take it from me, I'm definitely sure you should do what I say" advice before you've asked questions to better your knowledge. It's like Readyyyyy… FIRE!!! Aimmmmm…

Problem is that nobody knows when they do know everything there is to know ... or don't. That applies to everybody.

I would've just left it as-is, given the close fit of the drill bit.