Found it in a 1973 Motors Manual in the Carter carb section. Says that type of carb was used on Pontiac and Lincoln. It's an Idle Air Screw. States that this carb doesn't have the usual idle speed screw and uses this for that function. You turn the screw outward to increase idle speed, this leans out the idle circuit and the two idle mixture need to be adjusted at the same time to get the idle speed and mixture correct.
ps. Looks like Tom and I found it at the same time