Correct me if I'm wrong, but don't the '68 fenders just have a hole? There's no flange, or rolled edge or anything. Just a round hole with a small indent in the hole to index the light.
What I'm getting at is there would be pretty much no way to tell if the hole was factory or added later unless it was added in the wrong spot or poorly executed.A stamped hole and a hole saw cut hole with a smoothed edge prepped for paint look the same, and the indexing mark can be added carefully. And yes those are '68 sidemarkers on the front of my Duster. Soon to be the in the rear too. It's very simple to add them.
Unless you've held your fender in bare metal up next to an original '68 in bare metal it would be nearly impossible to tell if the job was done well. Maybe you could use a set of calipers do measure a handful of '68 marker holes and see if they're the same size, a stamped hole can be any size while the cut hole would at least start a standard size. Although that assumes all the original marker holes are very close to the same size, which they might not be, especially over the entire production run.
And, if the car wasn't purchased until December '68 it would have been on the lot quite awhile. Maybe an enterprising young employee decided that leftover '67 could be made to match the '68's? Maybe a buyer liked the new '68 lights and wanted them on that leftover '67 but the sale fell through? Dealers added a lot of stuff back then.
The production lines ran into July sometimes late into July before things stopped for retooling. A late June build having '68 markers would mean there would still be at least a couple, if not several weeks of production being built like that. Meaning, a lot more then just one car would be like that if the factory had started using the '68 markers early.
