I have an estimated Dearborn Build Date 23A that has the Bumpers with the Screw Holes. The same car has Wiper Bezels, Screw on Windshield and Backlight Molding Clips, an 'A' on the Air Vent Knob and other 64-1/2 features.
I also had a Dearborn 28R with Plain Bumpers.
Since my current car was 5 months into the 1965 production year and still had 64-1/2 elements, there may still have been some old bumpers that made it onto your car. Purely speculative mind you since I have nothing documented to show that this could be true. Plain seems more likely but again just speculation.
Ray