It really depends on the reflector. I've had 2 center reflectors side by side with totally different screw configurations.
Some have the bottom holes open for screws, some have soft pads for the plate to rest on but no holes. The upper holes can also have different sizes.
Just try something until it fits. There was also a license plate mounting repair kit which consisted of a bracket that fits into the original holes and the plate mounts on the bracket. The original holes get drilled larger before the bracket is inserted. It's a Ford OEM kit so you should be able to pick it up from the dealer if you can't get something to fit. It works for all year Contours and Mystiques and has instructions for each style.
Also, 6x1 doesn't fit (too big), so I just did what I didn't want to do (the cheap way), and that is re-tap the brass threads. Oh well, they were pretty mangled anyway from trying different sizes.
Everything is good now, and the car passed its e-test today with flying colours :laugh: