[Tom Vaught]

If you look on the End Carbs Illustration you will see that the 6-pack carbs had idle mixture screws on the Secondary long before the first 4-Corner Idle 4-BBLs were introduced.

The reason for the mixture screws is because we do not have a common base where we could machine in "Purge Slots" like a 4 BBL on the Vacuum Secondary carbs.
The rare Mechanical Secondary Carbs had a Metering Block and Idle Circuits just like the Primary side of any 2 BBL.

Chrysler wanted to be able to have the same Air/Fuel Mixture to each cylinder at idle with their 6-Pack Manifolds so they did dymo testing (as I mentioned before with the manifolds and tweeked the mixture screws on the end carbs until all of the runners were getting the same Air/Fuel Mixture with THAT manifold.
Then they had Holley set that Air/Fuel Ratio on that List # carb on the flow stands in production. Then they PLUGGED the screws so they could not be adjusted without destroying the plugs/base around the plugs.

Many racers did drill out the plugs and mess with the adjustments for racing, most times making the cylinder to cylinder balance a mess at idle but they were RACE CARBS AND ENGINES! Right. You have to screw with a Holley carb or you are not a racer.
(Have posted before how happy some owners are after I put the calibration back to stock Holley settings. So that is why the plugs and mixture screws are there.

Tom V.