I have an old Brownells Parts and Assembly book that has all the screw sizes for a Parker. According to this reference the trigger plate screws are supposed to be 8/36, I measured an old screw and the thread was .168 OD. If the screws that Bill had were 10/36 it would explain the reason they would not fit. This screw should be easy to make on a lathe.

I learn something every day, and a lot of times it's that what I learned the day before was wrong