The reason this article is being written is because of the ease of installing these ornaments upside down as observed in various pictures that appear in the Early bird as well as other places.
FoMoCo referred to these parts as ornaments part number 16228-A right side and 16229-A left side. Thunderbird owners sometimes refer to them as louvers or hash marks. They are not interchangeable from left to right fenders, but they can be installed upside down on each fender because of the casting and alignment of the mount studs. Think of these ornaments as being louvers that exit toward the rear of the car.
Photo #1 is from an original FoMoCo Trim and Sealer manual page 7-3556 showing the correct position for installation on the right front fender, the right side is the only drawing in the trim and sealer manual.
Another way to determine proper installation is the top of the louvers have a straight connecting casting between the louvers.