That's it in a nut shell. Bolt for bolt they are the same except for the reflector cut outs.
That's not true.
The 69 fender has an extra part welded to it that bolts to the top of the radiator support, the same part holds the hood adjuster bumper stud. The 68 fender doesn't have that part as it is part of the structure.....If you are using a 69 fender on a 68 Charger you can simply remove that part ,but if you are going the other way you will need to weld it to the replacement 68 fender. And then swap out the s/m light openings for the proper year.