1968 gts avs carburetor

Nah, The idle mixture screws are recessed back in the holes below(lower than) the aluminum caps that you referenced above. The screw in the middle is just an adjustable air bleed. Shine a light in the holes in the bottom. The screws are brass and possibly frozen. Although, someone has used them because they have lead plugs over them from the factory. If yours are frozen, take it off, drain it good and blow it out with air. Your gonna add fire, so get rid of as much of the flammable stuff as possible.
Now tilt it face up on your bench, take your burnzomatic and heat the baseplate directly below the screws. Get it nice and hot like 60 seconds per screw. Now, squirt some solvent in the holes. It may smoke and evaporate at first, wait a couple seconds, then squirt again. Do this until it puddles on top of the screws. Now, try wiggling the screws (don't horse them). If they're not free, reheat and fill the holes with solvent and walk away until the next day.

DISCLAIMER: Don't use anything super flammable:D