What is this switch?

Thank for the diagram, appreciate it. I think I know the problem now.

Found C2A-18DBL hang around the alternator, plug it into the A/C compressor wire: the clutch engaged. That mean I just by-pass the AC low pressure switch. The C2A-18DBL should go to the AC low pressure switch, and another wire go from switch to compressor.

Im gonna replace all o-ring and gasket, who sale these gasket set? I keep the R12, if that make a different.

The car is a restored 72 GTX.