If that hole is perpendicular to the center of the driveshaft, then my best guess is that it is threaded and that is a hex drive set screw. Soak it with rust solvent and first try a hex wrench. If that doesn't work, try an easy out in a slow drill press (in reverse, of course). Last resort is to drill it out and retap with a larger set screw. My PHD had a diagonal roll pin which caught the groove on the input shaft, and had 2 holes to allow it to be driven out. The fact that the face of that hole is machined, makes me think it is tapped.