My guess would be a vent.

 

Bowl vent

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I've never seen an AFB or AVS with anything there. Certainly the look of the main case and throttle body shape on that side would give the impression that it as designed with the thought of having an alternative accelerating pump location, and it obviously does function to equal up the float bowl volume side to side even though the left side does surrender some of that volume to the pump charge and the volumes of the pump shaft, piston, and spring.

Bogie

 

This may show the breakdown. http://www.mymopar.com/downloads/carterafbtuning.pdf

 

That AFB tuning manual is primarily for the 9000 series which doesn't have anything there. That's an excellent manual on tuning regardless.

A bowl vent, eh? As a guess? It could be but from what I can see it doesn't really strike me as being that. There's a shaft coming through the air horn, but where does the air vent through? Then there's a bore underneath it in the main body. Why would a bowl vent need a bore like that? I can't see the manufacturer casting and machining a bore in the body like that if it had no purpose other than to make the body look symmetrical from the outside. It looks like there was in some versions of the AFB an accelerator pump type plunger in there, but for what? Dual accelerator pumps? Obviously not really needed. It could be a bowl vent but that doesn't add up real well either. Nothing does.

 

Probably there as a engineered design for dual pumps, but then never utilized on production models. Lots of things get planned for in design stage that never come to fruition once reaching the production stage....this is true of all mechanical things.
Once the die casting mold was set, no sense in time or money making a new mold just to remove the cast area for a feature you didn't use...especially if it impacts nothing as far a other features or functions are concerned.

 

OK, I have the definitive answer on this (and yes, I got it from a friend who is much more up to speed on these than I am). The device on the front passenger side of the air horn is an internal dashpot. Apparently Ford used this on a lot of their factory AFBs in the 1950s and early 60s. The one in the photo is apparently from a 59 T-bird. The shaft sticking up is connected to a piston with holes in it that rides in a bore.

I thought this might be the case, as GM did something similar with the dashpot for the secondary air valves on the very early Qjets, as seen here. Note the arm that runs above the secondary air valve on this 1966 Qjet. The end has a link that connects to a fuel-damped piston in the fuel bowl that serves as the air valve dashpot. This was changed to the external vacuum dashpot in 1967.

 

An internal dashpot makes more sense than anything else I've seen or thought of. For throttle return do you suppose? Could you possibly get some pictures of the internals of that from your friend? That would be marvelous!

I had seen that internal dashpot for the secondaries on the early Q-jet too. What I had never seen was the bowl vent at the top front of the air horn like in your picture. I have always wondered what that blank boss area was for in my versions of the Q-jet.

OK, I have the definitive answer on this (and yes, I got it from a friend who is much more up to speed on these than I am). The device on the front passenger side of the air horn is an internal dashpot. Apparently Ford used this on a lot of their factory AFBs in the 1950s and early 60s. The one in the photo is apparently from a 59 T-bird. The shaft sticking up is connected to a piston with holes in it that rides in a bore.

 

Further research shows that device on a 67 Lincoln.
It’s called a ‘hot idle compensator.
Like in this link.
https://www.carburetor-parts.com/Hot-Idle-Compensator_ep_1844.html

 

Sorry, no. The hot idle compensator on an AFB is located in the main body between the secondary venturies.

 

I went digging in my old carburetor pile, actuallyI keep them as 1 sometimes 2 to cardboard box, been dragging these things from job assignment to jod assignment for nearly 60 years.

Well as luck has these things getting on toward the last box, lo and behold is an old 9000 with this vent. Looks to function as part of an emission system that would vent to a charcoal canister or some other vapor recovery system, it is linkage controlled and looks a lot like a hot idle compensator valve but flipped over in order to open or close access to a hose nipple. Out of the several Carters and Edelbrocks I have in storage this is the only one like that, being a junk yard find from what is a long time ago and from where who knows given the several places I've lived around the country.

Bogie

 

The later Carter carbs DID put a bowl vent there, but that vent has a tube that connects to the charcoal cannister on the 1970-later cars. The 1963 carb in the original photo did not use such a bowl vent.