What is the purpose of the circular metal sheet with a rectangular hole above the drain in the oil pan?
To hold any small bits in place so the oil does not pick them up.
When the flywheel and magnets are spinning in the oil, a tremendous current is created that can suspend debris, trash, small steel fasteners, cotter pins, nuts and washers that may have dislodged or fallen in pan and distribute them throughout the engine. That piece is meant to capture and hold the debris in the bottom, shielded from the turmoil, until the next oil change when the plug is removed and the debris can flow out. It works. One time I changed my oil and in the bottom of the bucket was a cotter pin and several pieces of wire that I did not know was in there. Jim Patrick.