308 GTS 1979 Calipers rear HELP

Can someone explain the Threaded stud on the inside of the rear calipers.

What is their purpose? Where can I buy them?

The threaded stud is the Pad to Rotor "adjusting screw".

Once the pistons are backed all the way out, as when replacing worn pads, then, once the new pads are installed the srews are turned reverse to push the piston out to achieve the correct Pad to Rotor distance. This distance is measured with a feeler gauge slid between the pad & rotor. The correct pad to rotor distance is 0.005" - 0.007" when the feeler gauge can be pulled out with just slight resistance. Then the operation is repeated for the other pad. I tighten up one pad very slightly to prevent any rotor deflection so I get accuracy on the side I'm adjusting. Then on the oppisite side after the first side is completed and lock nut tightened I use as little pressure as I can so that I also achieve the correct distance for that pad.

Where can you buy a new screw, I have no idea, but I think Board Advertiser AW Import Parts sells rebuilt Calipers, complete? So you might want to give him a call and see if he has any spare parts.